diff --git a/VERSION b/VERSION
index 7ec1d6d..ccbccc3 100644
--- a/VERSION
+++ b/VERSION
@@ -1 +1 @@
-2.1.0
+2.2.0
diff --git a/exabel_data_sdk/client/api/entity_api.py b/exabel_data_sdk/client/api/entity_api.py
index e772c07..1e7acef 100644
--- a/exabel_data_sdk/client/api/entity_api.py
+++ b/exabel_data_sdk/client/api/entity_api.py
@@ -146,6 +146,36 @@ def update_entity(self, entity: Entity, update_mask: FieldMask = None) -> Entity
         )
         return Entity.from_proto(response)
 
+    def upsert_entity(self, entity: Entity, assume_exists: bool = True) -> Entity:
+        """
+        Upsert an entity.
+
+        If the entity already exists, update it by replacement. Otherwise, create it.
+
+        Args:
+            entity:         The entity to upsert.
+            assume_exists:  If True, the entity is assumed to exist. Will try to to update
+                            the entity, and fall back to creating it if it does not exist,
+                            and vice versa.
+        """
+        if assume_exists:
+            try:
+                entity = self.update_entity(entity)
+            except RequestError as error:
+                if error.error_type == ErrorType.NOT_FOUND:
+                    entity = self.create_entity(entity, entity.get_entity_type())
+                else:
+                    raise error
+        else:
+            try:
+                entity = self.create_entity(entity, entity.get_entity_type())
+            except RequestError as error:
+                if error.error_type == ErrorType.ALREADY_EXISTS:
+                    entity = self.update_entity(entity)
+                else:
+                    raise error
+        return entity
+
     def delete_entity(self, name: str) -> None:
         """
         Delete one entity.
@@ -189,6 +219,7 @@ def bulk_create_entities(
         entities: Sequence[Entity],
         entity_type: str,
         threads: int = 40,
+        upsert: bool = False,
     ) -> ResourceCreationResults[Entity]:
         """
         Check if the provided entities exist, and create them if they don't.
@@ -197,6 +228,10 @@ def bulk_create_entities(
         """
 
         def insert(entity: Entity) -> ResourceCreationStatus:
+            if upsert:
+                # Upsert entities assuming they already exist.
+                self.upsert_entity(entity=entity, assume_exists=True)
+                return ResourceCreationStatus.UPSERTED
             # Optimistically insert the entity.
             # If the entity already exists, we'll get an ALREADY_EXISTS error from the backend,
             # which is handled appropriately by the bulk_insert function.
diff --git a/exabel_data_sdk/client/api/relationship_api.py b/exabel_data_sdk/client/api/relationship_api.py
index 7f980af..a268dc3 100644
--- a/exabel_data_sdk/client/api/relationship_api.py
+++ b/exabel_data_sdk/client/api/relationship_api.py
@@ -243,6 +243,38 @@ def update_relationship(
         )
         return Relationship.from_proto(response)
 
+    def upsert_relationship(
+        self, relationship: Relationship, assume_exists: bool = True
+    ) -> Relationship:
+        """
+        Upsert a relationship between two entities.
+
+        If the relationship already exists, update it by replacement. Otherwise, create it.
+
+        Args:
+            relationship:   The relationship to upsert.
+            assume_exists:  If True, the relationship is assumed to exist. Will try to to update
+                            the relationship, and fall back to creating it if it does not exist,
+                            and vice versa.
+        """
+        if assume_exists:
+            try:
+                relationship = self.update_relationship(relationship)
+            except RequestError as error:
+                if error.error_type == ErrorType.NOT_FOUND:
+                    relationship = self.create_relationship(relationship)
+                else:
+                    raise error
+        else:
+            try:
+                relationship = self.create_relationship(relationship)
+            except RequestError as error:
+                if error.error_type == ErrorType.ALREADY_EXISTS:
+                    relationship = self.update_relationship(relationship)
+                else:
+                    raise error
+        return relationship
+
     def delete_relationship(self, relationship_type: str, from_entity: str, to_entity: str) -> None:
         """
         Delete a relationship.
@@ -277,6 +309,7 @@ def bulk_create_relationships(
         self,
         relationships: Sequence[Relationship],
         threads: int = 40,
+        upsert: bool = False,
     ) -> ResourceCreationResults[Relationship]:
         """
         Check if the provided relationships exist, and create them if they don't.
@@ -284,6 +317,10 @@ def bulk_create_relationships(
         """
 
         def insert(relationship: Relationship) -> ResourceCreationStatus:
+            if upsert:
+                # Upsert relationships assuming they already exist.
+                self.upsert_relationship(relationship=relationship, assume_exists=True)
+                return ResourceCreationStatus.UPSERTED
             # Optimistically insert the relationship.
             # If the relationship already exists, we'll get an ALREADY_EXISTS error from the
             # backend, which is handled appropriately by the bulk_insert function.
diff --git a/exabel_data_sdk/client/api/resource_creation_result.py b/exabel_data_sdk/client/api/resource_creation_result.py
index f66f5ed..da54749 100644
--- a/exabel_data_sdk/client/api/resource_creation_result.py
+++ b/exabel_data_sdk/client/api/resource_creation_result.py
@@ -26,6 +26,9 @@ class ResourceCreationStatus(Enum):
     # Denotes that creation failed.
     FAILED = 3
 
+    # Denotes that a resource was upserted.
+    UPSERTED = 4
+
 
 class ResourceCreationResult(Generic[TResource]):
     """
@@ -57,6 +60,7 @@ def __init__(
             print_status:    Whether to print status of the upload during processing.
             abort_threshold: If the fraction of failed requests exceeds this threshold,
                              the upload is aborted.
+            upsert:          Update resources that already exist.
         """
         self.results: List[ResourceCreationResult[TResource]] = []
         self.counter: Counter = Counter()
@@ -117,9 +121,12 @@ def check_failures(self) -> None:
 
     def print_summary(self) -> None:
         """Prints a human legible summary of the resource creation results to screen."""
-        print(self.counter[ResourceCreationStatus.CREATED], "new resources created")
+        if self.counter[ResourceCreationStatus.CREATED]:
+            print(self.counter[ResourceCreationStatus.CREATED], "new resources created")
         if self.counter[ResourceCreationStatus.EXISTS]:
             print(self.counter[ResourceCreationStatus.EXISTS], "resources already existed")
+        if self.counter[ResourceCreationStatus.UPSERTED]:
+            print(self.counter[ResourceCreationStatus.UPSERTED], "resources upserted")
         if self.counter[ResourceCreationStatus.FAILED]:
             print(self.counter[ResourceCreationStatus.FAILED], "resources failed:")
             for result in self.results:
@@ -134,13 +141,17 @@ def print_status(self) -> None:
         Note that the previous status message is overwritten (by writing '\r'),
         but this only works if nothing else has been printed to stdout since the last update.
         """
+        message_parts = []
         fraction_complete = self.count() / self.total_count
-        sys.stdout.write(
-            f"\r{fraction_complete:.0%} - "
-            f"{self.count(ResourceCreationStatus.CREATED)} created, "
-            f"{self.count(ResourceCreationStatus.EXISTS)} exists, "
-            f"{self.count(ResourceCreationStatus.FAILED)} failed"
-        )
+        message_parts.append(f"\r{fraction_complete:.0%} - ")
+        if self.counter[ResourceCreationStatus.CREATED]:
+            message_parts.append(f"{self.count(ResourceCreationStatus.CREATED)} created, ")
+        if self.counter[ResourceCreationStatus.UPSERTED]:
+            message_parts.append(f"{self.count(ResourceCreationStatus.UPSERTED)} upserted, ")
+        if self.counter[ResourceCreationStatus.EXISTS]:
+            message_parts.append(f"{self.count(ResourceCreationStatus.EXISTS)} exists, ")
+        message_parts.append(f"{self.count(ResourceCreationStatus.FAILED)} failed")
+        sys.stdout.write("".join(message_parts))
         if fraction_complete == 1:
             sys.stdout.write("\n")
         sys.stdout.flush()
diff --git a/exabel_data_sdk/client/api/time_series_api.py b/exabel_data_sdk/client/api/time_series_api.py
index c695144..138ab2b 100644
--- a/exabel_data_sdk/client/api/time_series_api.py
+++ b/exabel_data_sdk/client/api/time_series_api.py
@@ -338,7 +338,7 @@ def insert(ts: pd.Series) -> ResourceCreationStatus:
             existed = self.upsert_time_series(
                 str(ts.name), ts, create_tag=create_tag, default_known_time=default_known_time
             )
-            return ResourceCreationStatus.EXISTS if existed else ResourceCreationStatus.CREATED
+            return ResourceCreationStatus.UPSERTED if existed else ResourceCreationStatus.CREATED
 
         return bulk_insert(series, insert, threads=threads)
 
diff --git a/exabel_data_sdk/scripts/load_entities_from_csv.py b/exabel_data_sdk/scripts/load_entities_from_csv.py
index 59ee011..e617121 100644
--- a/exabel_data_sdk/scripts/load_entities_from_csv.py
+++ b/exabel_data_sdk/scripts/load_entities_from_csv.py
@@ -57,6 +57,13 @@ def __init__(self, argv: Sequence[str], description: str):
             help="The column name for the entity description. "
             "If not specified, no description is provided.",
         )
+        self.parser.add_argument(
+            "--upsert",
+            required=False,
+            action="store_true",
+            default=False,
+            help="Update entities if they already exist.",
+        )
 
     def run_script(self, client: ExabelClient, args: argparse.Namespace) -> None:
 
@@ -101,7 +108,9 @@ def run_script(self, client: ExabelClient, args: argparse.Namespace) -> None:
             return
 
         try:
-            client.entity_api.bulk_create_entities(entities, entity_type_name, threads=args.threads)
+            client.entity_api.bulk_create_entities(
+                entities, entity_type_name, threads=args.threads, upsert=args.upsert
+            )
         except BulkInsertFailedError:
             # An error summary has already been printed.
             pass
diff --git a/exabel_data_sdk/scripts/load_relationships_from_csv.py b/exabel_data_sdk/scripts/load_relationships_from_csv.py
index 4c252fd..edb6540 100644
--- a/exabel_data_sdk/scripts/load_relationships_from_csv.py
+++ b/exabel_data_sdk/scripts/load_relationships_from_csv.py
@@ -65,6 +65,13 @@ def __init__(self, argv: Sequence[str], description: str):
             help="The column name for the relationship description. "
             "If not specified, no description is provided.",
         )
+        self.parser.add_argument(
+            "--upsert",
+            required=False,
+            action="store_true",
+            default=False,
+            help="Update relationships if they already exist.",
+        )
 
     def run_script(self, client: ExabelClient, args: argparse.Namespace) -> None:
 
@@ -130,7 +137,9 @@ def run_script(self, client: ExabelClient, args: argparse.Namespace) -> None:
             return
 
         try:
-            client.relationship_api.bulk_create_relationships(relationships, threads=args.threads)
+            client.relationship_api.bulk_create_relationships(
+                relationships, threads=args.threads, upsert=args.upsert
+            )
         except BulkInsertFailedError:
             # An error summary has already been printed.
             pass
diff --git a/exabel_data_sdk/tests/client/api/mock_entity_api.py b/exabel_data_sdk/tests/client/api/mock_entity_api.py
index 3207a5f..d01811f 100644
--- a/exabel_data_sdk/tests/client/api/mock_entity_api.py
+++ b/exabel_data_sdk/tests/client/api/mock_entity_api.py
@@ -44,7 +44,8 @@ def create_entity(self, entity: Entity, entity_type: str) -> Entity:
         return self.entities.create(entity)
 
     def update_entity(self, entity: Entity, update_mask: FieldMask = None) -> Entity:
-        raise NotImplementedError()
+        # Note: The mock implementation ignores update_mask
+        return self.entities.update(entity)
 
     def delete_entity(self, name: str) -> None:
         # Note: The mock implementation does not delete associated time series and relationships
diff --git a/exabel_data_sdk/tests/client/api/mock_relationship_api.py b/exabel_data_sdk/tests/client/api/mock_relationship_api.py
index fa160db..58246ff 100644
--- a/exabel_data_sdk/tests/client/api/mock_relationship_api.py
+++ b/exabel_data_sdk/tests/client/api/mock_relationship_api.py
@@ -76,7 +76,8 @@ def create_relationship(self, relationship: Relationship) -> Relationship:
     def update_relationship(
         self, relationship: Relationship, update_mask: FieldMask = None
     ) -> Relationship:
-        raise NotImplementedError()
+        # Note: The mock implementation ignores update_mask
+        return self.relationships.update(relationship, self._key(relationship))
 
     def delete_relationship(self, relationship_type: str, from_entity: str, to_entity: str) -> None:
         self.relationships.delete((relationship_type, from_entity, to_entity))
diff --git a/exabel_data_sdk/tests/client/api/mock_resource_store.py b/exabel_data_sdk/tests/client/api/mock_resource_store.py
index b72ff29..7c10b18 100644
--- a/exabel_data_sdk/tests/client/api/mock_resource_store.py
+++ b/exabel_data_sdk/tests/client/api/mock_resource_store.py
@@ -64,6 +64,26 @@ def create(self, resource: TResource, key: object = None) -> TResource:
         self.resources[key] = resource
         return resource
 
+    @failure_prone
+    def update(self, resource: TResource, key: object = None) -> TResource:
+        """
+        Update the given resource in the store.
+
+        Typically, resources have a resource name, which should be used as the key.
+        In this case, the key parameter should not be set.
+        If not, as in the case of Relationship resources, an explicit key must be provided.
+
+        Args:
+            resource: the resource to create in the store
+            key:      the key of the resource. Defaults to the resource's name.
+        """
+        if key is None:
+            key = resource.name  # type: ignore[attr-defined]
+        if key not in self.resources:
+            raise RequestError(ErrorType.NOT_FOUND, f"Does not exist: {key}")
+        self.resources[key] = resource
+        return resource
+
     def delete(self, key: object):
         """Delete the resource with the given key."""
         if key in self.resources:
diff --git a/exabel_data_sdk/tests/client/api/test_entity_api.py b/exabel_data_sdk/tests/client/api/test_entity_api.py
new file mode 100644
index 0000000..d409790
--- /dev/null
+++ b/exabel_data_sdk/tests/client/api/test_entity_api.py
@@ -0,0 +1,38 @@
+import unittest
+
+from exabel_data_sdk.client.api.data_classes.entity import Entity
+from exabel_data_sdk.client.api.entity_api import EntityApi
+from exabel_data_sdk.tests.client.api.mock_entity_api import MockEntityApi
+
+
+class TestEntityApi(unittest.TestCase):
+    def test_upsert(self):
+        for assume_exists in (True, False):
+            entity_api: EntityApi = MockEntityApi()
+            expected = Entity(
+                name="entityTypes/company/entities/Amazon",
+                display_name="Amazon",
+            )
+            created_entity = entity_api.upsert_entity(expected, assume_exists)
+            self.assertEqual(expected, created_entity)
+            updated_entity = entity_api.upsert_entity(expected, assume_exists)
+            self.assertEqual(expected, updated_entity)
+
+    def test_upsert_replaces_resource(self):
+        for assume_exists in (True, False):
+            entity_api: EntityApi = MockEntityApi()
+            old_entity = Entity(
+                name="entityTypes/company/entities/Amazon",
+                display_name="Amazon's old display name",
+                description="Amazon's old description",
+                properties={"old_property": "old_value"},
+            )
+            expected = Entity(
+                name="entityTypes/company/entities/Amazon",
+                display_name="Amazon",
+                description="Amazon's new description",
+            )
+            entity_api.create_entity(old_entity, old_entity.get_entity_type())
+            entity_api.upsert_entity(expected, assume_exists)
+            actual_entity = entity_api.get_entity(expected.name)
+            self.assertEqual(expected, actual_entity)
diff --git a/exabel_data_sdk/tests/client/api/test_relationship_api.py b/exabel_data_sdk/tests/client/api/test_relationship_api.py
new file mode 100644
index 0000000..9f4c219
--- /dev/null
+++ b/exabel_data_sdk/tests/client/api/test_relationship_api.py
@@ -0,0 +1,47 @@
+import unittest
+
+from exabel_data_sdk.client.api.data_classes.relationship import Relationship
+from exabel_data_sdk.client.api.relationship_api import RelationshipApi
+from exabel_data_sdk.tests.client.api.mock_relationship_api import MockRelationshipApi
+
+AMAZON = "entityTypes/company/entities/Amazon"
+USA = "entityTypes/country/entities/USA"
+RELATIONHIP_TYPE = "relationshipTypes/LOCATED_IN"
+
+
+class TestRelationshipApi(unittest.TestCase):
+    def test_upsert(self):
+        for assume_exists in (True, False):
+            relationship_api: RelationshipApi = MockRelationshipApi()
+            expected = Relationship(
+                relationship_type=RELATIONHIP_TYPE,
+                from_entity=AMAZON,
+                to_entity=USA,
+            )
+            created_relationship = relationship_api.upsert_relationship(expected, assume_exists)
+            self.assertEqual(expected, created_relationship)
+            updated_relationship = relationship_api.upsert_relationship(expected, assume_exists)
+            self.assertEqual(expected, updated_relationship)
+
+    def test_upsert_replaces_resource(self):
+        for assume_exists in (True, False):
+            relationship_api: RelationshipApi = MockRelationshipApi()
+            old_relationship = Relationship(
+                relationship_type=RELATIONHIP_TYPE,
+                from_entity=AMAZON,
+                to_entity=USA,
+                description="Old relationship description",
+                properties={"old_property": "old_value"},
+            )
+            expected = Relationship(
+                relationship_type=RELATIONHIP_TYPE,
+                from_entity=AMAZON,
+                to_entity=USA,
+                description="New relationship description",
+            )
+            relationship_api.create_relationship(old_relationship)
+            relationship_api.upsert_relationship(expected, assume_exists)
+            actual_relationship = relationship_api.get_relationship(
+                expected.relationship_type, expected.from_entity, expected.to_entity
+            )
+            self.assertEqual(expected, actual_relationship)
diff --git a/exabel_data_sdk/tests/resources/data/entities.csv b/exabel_data_sdk/tests/resources/data/entities.csv
index d6646dc..620ef47 100644
--- a/exabel_data_sdk/tests/resources/data/entities.csv
+++ b/exabel_data_sdk/tests/resources/data/entities.csv
@@ -1,4 +1,4 @@
 brand,description
 Spring & Vine,Shampoo bars
 The Coconut Tree,Sri Lankan street food
-Spring & Vine,This entry will be ignored because it's a duplicate
+Spring & Vine,This entry might be ignored because it's a duplicate
diff --git a/exabel_data_sdk/tests/resources/data/relationships.csv b/exabel_data_sdk/tests/resources/data/relationships.csv
index 60b99ce..6e4355f 100644
--- a/exabel_data_sdk/tests/resources/data/relationships.csv
+++ b/exabel_data_sdk/tests/resources/data/relationships.csv
@@ -1,4 +1,4 @@
 entity_from,brand,description
 entityTypes/company/company_x,Spring & Vine,Owned since 2019
-entityTypes/company/company_x,Spring & Vine,This entry will be ignored because it's a duplicate
+entityTypes/company/company_x,Spring & Vine,This entry might be ignored because it's a duplicate
 entityTypes/company/company_y,The Coconut Tree,Acquired for $200M
diff --git a/exabel_data_sdk/tests/scripts/common_utils.py b/exabel_data_sdk/tests/scripts/common_utils.py
index 8678dce..37c7231 100644
--- a/exabel_data_sdk/tests/scripts/common_utils.py
+++ b/exabel_data_sdk/tests/scripts/common_utils.py
@@ -5,10 +5,12 @@
 from exabel_data_sdk.tests.client.exabel_mock_client import ExabelMockClient
 
 
-def load_test_data_from_csv(csv_script: Type[CsvScript], args: Sequence[str]) -> ExabelClient:
+def load_test_data_from_csv(
+    csv_script: Type[CsvScript], args: Sequence[str], client: ExabelClient = None
+) -> ExabelClient:
     """Loads entities to an ExabelMockClient using exabel_data_sdk.scripts.load_entities_from_csv"""
     script = csv_script(args, f"Test{type(csv_script).__name__}")
-    client = ExabelMockClient()
+    client = client or ExabelMockClient()
     script.run_script(client, script.parse_arguments())
 
     return client
diff --git a/exabel_data_sdk/tests/scripts/test_load_entities_from_csv.py b/exabel_data_sdk/tests/scripts/test_load_entities_from_csv.py
index 0839cbf..97efa1e 100644
--- a/exabel_data_sdk/tests/scripts/test_load_entities_from_csv.py
+++ b/exabel_data_sdk/tests/scripts/test_load_entities_from_csv.py
@@ -76,3 +76,28 @@ def test_read_file_random_errors(self):
             for letter in "ABCDEFGHIJ"
         ]
         self.check_entities(client, expected_entities)
+
+    def test_read_file_with_upsert(self):
+        args = common_args + [
+            "--filename",
+            "./exabel_data_sdk/tests/resources/data/entities.csv",
+            "--description-col",
+            "description",
+            "--upsert",
+        ]
+        client = load_test_data_from_csv(LoadEntitiesFromCsv, args)
+        expected_entities = [
+            Entity(
+                name="entityTypes/brand/entities/test.Spring_Vine",
+                display_name="Spring & Vine",
+                description="This entry might be ignored because it's a duplicate",
+            ),
+            Entity(
+                name="entityTypes/brand/entities/test.The_Coconut_Tree",
+                display_name="The Coconut Tree",
+                description="Sri Lankan street food",
+            ),
+        ]
+        self.check_entities(client, expected_entities)
+        client = load_test_data_from_csv(LoadEntitiesFromCsv, args, client)
+        self.check_entities(client, expected_entities)
diff --git a/exabel_data_sdk/tests/scripts/test_load_relationships_from_csv.py b/exabel_data_sdk/tests/scripts/test_load_relationships_from_csv.py
index 904ea7a..d24d4d7 100644
--- a/exabel_data_sdk/tests/scripts/test_load_relationships_from_csv.py
+++ b/exabel_data_sdk/tests/scripts/test_load_relationships_from_csv.py
@@ -83,3 +83,37 @@ def check_relationships(self, client, expected_relationships):
                 expected_relationship.to_entity,
             )
             self.assertEqual(expected_relationship, relationship)
+
+    def test_read_file_with_upsert(self):
+        args = common_args + [
+            "--filename",
+            "./exabel_data_sdk/tests/resources/data/relationships.csv",
+            "--entity-from-column",
+            "entity_from",
+            "--description-column",
+            "description",
+            "--upsert",
+        ]
+        client = load_test_data_from_csv(LoadRelationshipsFromCsv, args)
+        # Check that the relationship type was created
+        self.assertEqual(
+            RelationshipType("relationshipTypes/acme.PART_OF"),
+            client.relationship_api.get_relationship_type("relationshipTypes/acme.PART_OF"),
+        )
+        expected_relationships = [
+            Relationship(
+                relationship_type="relationshipTypes/acme.PART_OF",
+                from_entity="entityTypes/company/company_x",
+                to_entity="entityTypes/brand/entities/acme.Spring_Vine",
+                description="This entry might be ignored because it's a duplicate",
+            ),
+            Relationship(
+                relationship_type="relationshipTypes/acme.PART_OF",
+                from_entity="entityTypes/company/company_y",
+                to_entity="entityTypes/brand/entities/acme.The_Coconut_Tree",
+                description="Acquired for $200M",
+            ),
+        ]
+        self.check_relationships(client, expected_relationships)
+        client = load_test_data_from_csv(LoadRelationshipsFromCsv, args, client)
+        self.check_relationships(client, expected_relationships)