Any user can edit their own posts, but only their own posts. To edit a post first make sure you are signed in. Then navigate to the post and look for the 'edit post' link. Click the link and you can change the subject as well as the body of the message. Post editing is great for fixing silly typos and the like.
Below you'll see the edit post link.
After clicking you'll see the form fields open.
When you're done with the post change you can see when the post was edited.