当使用WordPress插件WP User Frontend提交帖子时，如何使用functions.php更改帖子标题？

I am trying to auto-change the post title of a WP User Frontend submitted post by adding some code into the functions.php file.

Below is the code that I've added into functions.php file, but no luck, it doesn't work.

add_action( 'wpuf_add_post_after_insert', 'update_title' );
function update_title($title) {
   global $post;
   if (isset($post->ID)) {
      if (empty($_POST['post_title']) && 'post' == get_post_type($post->ID)){
         $id = get_the_ID();
         $title = 'Tuition Job - '.$id;} }
   return $title; 
}

For example, I expect the post title to change to "Tuition Job - 2343" when someone submits a post via the WP User Frontend plugin, but nothing happened.