If you are using Team Weeks as the estimation system and try to PATCH update the "ManWeeks" field in an Epic using the API, you will get a 204 No Content response and the field is not updated.  The field is marked as "Read Only" in the Swagger documentation, so this seems expected.

However, if you are using T-Shirt estimation and do a PATCH to update the T-Shirt size (also marked as read only in Swagger), the API call returns a 204 but does update the T-Shirt size on the Epic.

The "read-only" flags seem to function differently for these 2 estimation types.  If it is possible to update T-Shirt size using the API then it should also work for ManWeeks.  Please consider changing the API to allow changing ManWeeks with the API.