When you configure a synchronization task to perform upserts on a Salesforce target, configure the upsert field on the Schedule page of the Synchronization Task wizard. You can use an ID field for standard objects. You can use external ID field or any field with the idLookup field property enabled for standard and custom objects. Ensure that you include the upsert field in the field mappings for the task.
Note: The task fails if the Salesforce user account defined does not have creatable or updatable permissions on the external ID field or any field with the idLookup field property enabled. When you configure the upsert task operation with an external ID or idLookup field that is write-protected, ensure that the data exists.