def METHOD_NAME(request: HttpRequest, password: str) -> Optional[str]:
    """
    Set the user's password. If anything is amiss, return a string
    describing the error; otherwise, return None.
    """
    req_user_id = request.session.get(USER_ID_SESSION_KEY)
    req_ts = request.session.get(VERIFIED_TIMESTAMP_SESSION_KEY, 0)
    now = time.time()
    time_elapsed = now - req_ts
    if req_user_id is None or time_elapsed > NEW_PASSWORD_MAX_SECS:
        return "Please go back and re-enter your phone number."
    user = JustfixUser.objects.get(pk=req_user_id)
    user.METHOD_NAME(password)
    user.save()
    logger.info(f"User {user.username} has changed their password.")
    slack.sendmsg_async(
        f"{slack.hyperlink(text=user.best_first_name, href=user.admin_url)} "
        f"has changed their password.",
        is_safe=True,
    )
    del request.session[USER_ID_SESSION_KEY]
    del request.session[VERIFIED_TIMESTAMP_SESSION_KEY]
    return None