At the bottom right hand corner of your post, these are three little boxes next to the "Add a reply" button. You can click on either of the first two to add more information to your original post.
■The first one is "Edit this message", which brings up the whole message to be edited as a full edit.
■The second on is the "Quick Edit", which brings up the message within the post and looks similar to the box for "Quick reply" at the bottom of the page.