code: def update(self, table, columns, values):
          
          self._mutations.append(Mutation(update=_make_write_pb(table, columns, values)))
docs: Update one or more existing table rows.
  
          :type table: str
          :param table: Name of the table to be modified.
  
          :type columns: list of str
          :param columns: Name of the table columns to be modified.
  
          :type values: list of lists
          :param values: Values to be modified.