how do i delete an old post

You need to contact the admins and moderators of that forum as only them can delete posts or whole threads.

my admin and moderators cant seem to delete posts.
where can i fing how to delete posts?

When an admin or mod is logged in, there is a 'Show Topic Admin Actions' button at the top right of a topic, clicking this will drop down options for deleting and moving posts.

If they are not seeing that button, they should make sure they are logged in correctly.