I'm pretty sure it can't be done. I've seen other people say that they tried and it wouldn't allow them.
Just click edit and go advanced. As long as the thread's not old it'll let you I believe (haven't tried lately).
I believe you can edit your thread and title within the first eight hours of posting. If you need to edit after than use report bad post icon (triangle outlined in red) and request a mod edit it for you.