code: def update(self, reference, field_updates, option=None):
          
          if option.__class__.__name__ == "ExistsOption":
              raise ValueError("you must not pass an explicit write option to " "update.")
          write_pbs = _helpers.pbs_for_update(
              reference._document_path, field_updates, option
          )
          self._add_write_pbs(write_pbs)
docs: Add a "change" to update a document.
  
          See
          :meth:`~.firestore_v1beta1.document.DocumentReference.update` for
          more information on ``field_updates`` and ``option``.
  
          Args:
              reference (~.firestore_v1beta1.document.DocumentReference): A
                 document reference that will be deleted in this batch.
              field_updates (dict): Field names or paths to update and values
                  to update with.
              option (Optional[~.firestore_v1beta1.client.WriteOption]): A
                 write option to make assertions / preconditions on the server
                 state of the document before applying changes.