designtopack/public/site/plugins/comments/routes/update.php

<?php

return [
  'pattern' => '(:all)update-comment.json',
  'method' => 'POST',
  'action'  => function () {
      $json = file_get_contents('php://input');
      $data = json_decode($json);

      $page = page($data->pageUri);
      $file = $page->file($data->fileName);
      $user = kirby()->user($data->userUuid);


      $comments = $file->comments()->isEmpty() == true ? [] : Yaml::decode($file->comments()->value());

      $comments[$data->id]['text'] = $data->text;
      $comments[$data->id]['date'] = $data->date;
      
      $newFile = $file->update([
        'comments' => $comments
      ]);

      $user->sendNotification('comments', $comments[$data->id]);

      return getFileData($newFile);
  }
];