Updates a letter

        :param letter_id: the letter id
        :param letter_dict: dict
        :return: dict

def update_letter(self, letter_id, letter_dict):
        """
        Updates a letter

        :param letter_id: the letter id
        :param letter_dict: dict
        :return: dict
        """
        return self._create_put_request(
            resource=LETTERS,
            billomat_id=letter_id,
            send_data=letter_dict
        )