"content" represent pyWorkPage and you can update any object inside pyWorkPage.
However, I have a problem updating any page or page list which sits outside pyWorkPage or "content" like temporary pages that can be seen on a clipboard case thread but cannot be updated for instance with the following JSON structure.
I agree with PraneethPurighalla. It's not possible with the OOTB Cases REST API's. However, you can create an Activity which performs the task for you, wrap it in a Data Page and use the OOTB 'Get Data' REST API to execute the Data Page.