https://api.slack.com/methods/groups.rename

def rename(self, group_name, new_name):
        """ https://api.slack.com/methods/groups.rename
        """
        group_id = self.get_group_id(group_name)
        self.params.update({
            'channel': group_id,
            'name':    new_name,
            })
        return FromUrl('https://slack.com/api/groups.rename', self._requests)(data=self.params).post()