code: def modify_signature(self, signature):
          
          # if no content is specified, just use a selector (id/name)
          dic = signature.to_creator(for_modify=True)
  
          self.request('ModifySignature', {'signature': dic})
docs: Modify an existing signature
  
          Can modify the content, contenttype and name. An unset attribute will
          not delete the attribute but leave it untouched.
          :param: signature a zobject.Signature object, with modified
                           content/contentype/name, the id should be present and
                            valid, the name does not allows to identify the
                           signature for that operation.