如何编辑Wordpress post.php文件?

I have asked this question on WordPress StackExchange where it belongs but I usually don't get as much replies and activity on there as on this so I will post it here as well.

I have a Custom Post Type which contain a Gravity Form on my website. I would like to display a message on the back end of the Post when logged in as admin. The message will say when the form has reached its entry submission limit.

The file that displays the post content in the back end is the post.php file but I am not allowed to edit it directly. How would I go about achieving this functionality? I already know what I need to do to check if the form has reached it's limit but how would I have this replicated in the back end.