I already have form. With AJAX submit After submit I'm validating thing & showing message. But after success message, form is not reset. It keeps the old value

  public function ajaxSubmitForm(array &$form, FormStateInterface $form_state) {
    $response = new AjaxResponse();

    if ($form_state->hasAnyErrors()) {
      $response->addCommand(new ReplaceCommand('#modal-form', $form));
    }
    else {
      $form_values = $form_state->getValues();
      $form_state->setValue('first_name', '');
      $response->addCommand(new ReplaceCommand('#success-msg', "Success! Your message has been sent"));
    }

    return $response;
  }

Above is the code for submit form.