Change username
        :param username: email or str
        :return:

def change_username(self, username):
        """
        Change username
        :param username: email or str
        :return:
        """
        username = username.lower()
        if self.username != username:
            if self.get_by_username(username):
                raise exceptions.AuthError("Username exists already")
            self.update(username=username)