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
External endpoint content type
The URL specified in the
external_endpointproperty of the external details object, must return a
Content-Typeheader set to
application/jsonother content types will cause an error when creating the collection.
When changing external details, you should include all existing properties. Any missing properties in your request, will be deleted from the collection configuration.