Osapi Update Document MultipartBody

Hey,

can anybody provide a example call for the following api call please.

Jive JavaScript API v3.14 → osapi.jive.corev3.contents.Document entity

I am struggling to build the multipartbody parameter.

osapi.jive.corev3.documents.get({uri: "/contents/1231123"}).execute(function (response) {

  response.update(parameter).execute(function (response2) {

  });

});

For the beginning I would like to update the subject of the document.

Thank you in advance,

Sinan