code: def update_model(self, model, fields, retry=DEFAULT_RETRY):
          
          partial = model._build_resource(fields)
          if model.etag:
              headers = {"If-Match": model.etag}
          else:
              headers = None
          api_response = self._call_api(
              retry, method="PATCH", path=model.path, data=partial, headers=headers
          )
          return Model.from_api_repr(api_response)
docs: [Beta] Change some fields of a model.
  
          Use ``fields`` to specify which fields to update. At least one field
          must be provided. If a field is listed in ``fields`` and is ``None``
          in ``model``, it will be deleted.
  
          If ``model.etag`` is not ``None``, the update will only succeed if
          the model on the server has the same ETag. Thus reading a model with
          ``get_model``, changing its fields, and then passing it to
          ``update_model`` will ensure that the changes will only be saved if
          no modifications to the model occurred since the read.
  
          Args:
              model (google.cloud.bigquery.model.Model): The model to update.
              fields (Sequence[str]):
                  The fields of ``model`` to change, spelled as the Model
                  properties (e.g. "friendly_name").
              retry (google.api_core.retry.Retry):
                  (Optional) A description of how to retry the API call.
  
          Returns:
              google.cloud.bigquery.model.Model:
                  The model resource returned from the API call.