Update Old Upload Image in Laravel

PostController.php

<?php

use Illuminate\Support\Facades\Storage;

public function update(Post $post)
{
      if ( request()->file('thumbnail')) {
          Storage::delete($post->thumbnail);
          $thumbnail = request()->file('thumbnail')->store('images/posts' );
      }else{
          $thumbnail = $post->thumbnail;
      }

      $attr['thumbnail'] = $thumbnail;
      $post->update($attr);

      return back();
}