How to change the title of submit button when someone register in mailchimp?
As mentioned, you'll want to use hook_form_alter(). You can check if the user has subscribed with mailchimp_is_subscribed() and change the submit button text accordingly.

/**
 * Implements hook_form_alter().
 */
function my_module_form_alter(&amp;$form, &amp;$form_state, $form_id) {
  if ($form_id == 'mailchimp_lists_user_subscribe_form_test') {
    global $user;
    // First parameter is the list id, which can be found here:
    // https://polkaspots.zendesk.com/hc/en-us/articles/201070513-Where-can-I-find-my-MailChimp-API-key-and-List-ID
    $form['submit']['#value'] = mailchimp_is_subscribed('123abc', $user-&gt;mail) ? t('Unregister') : t('Register');
  }
}