How can I edit comment meta value before it is saved?

You can use “save_post” action-hook

Add the code below into functions.php and enchance with your comment_meta code.

function update_comments_meta( $post_id ) {
   // Do whatever add/update_comment_meta code you need
}
add_action( 'save_post', 'update_comments_meta' );

---

UPDATE.

As an example i’ve attach code below. It performs on post save/update action fired. Code gets current post comments (all) then looping through received comment object it launch update_comment_meta with ID extracted from comments object.

function update_comments_meta($post_id) {
    $args = array(
        'post_id' => $post_id,
    );
    $comments = get_comments($args);
    if (!empty($comments))
        foreach ($comments as $comment) {
            update_comment_meta($comment->comment_ID, 'YOUR_META_KEY', 'YOUR_META_VALUE');
        }
}

add_action('save_post', 'update_comments_meta');

Hope this is the case that you where interested in. Code tested on 4.3 twentyfifteen.

Refferences:

1. update_comment_meta()
2. get_comments()