How do I edit the Wordpress post.php file? -

i have asked question on wordpress stackexchange belongs don't replies , activity on there on post here well.

i have custom post type contain gravity form on website. display message on end of post when logged in admin. message when form has reached entry submission limit.

the file displays post content in end post.php file not allowed edit directly. how go achieving functionality? know need check if form has reached it's limit how have replicated in end.