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Only the collection settings can be updated with this endpoint. Updating a field of a collection can be done with the Fields API documented below

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External endpoint content type
The URL specified in the external_endpoint property of the external details object, must return a Content-Type header set to application/json other content types will cause an error when creating the collection.

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When changing external details, you should include all existing properties. Any missing properties in your request, will be deleted from the collection configuration.