You can Edit any post you have made... and even some you can Delete (thou sometimes the option is not there).
The Edit option appears at the end of your post, Right after yours nick and the time elapsed since publication
The inbox is just that, same as email. if you could edit the posts, then the logic behind it would have to be changed so that user can be informed that a conversation was edited, otherwise they will miss the changes you've made. Just like in the forum.