Unless I'm missing something, it appears that once you have posted a thread, you can't go back and edit the title You can edit the body text of the message, but not the thread title
On the "classic" forum, I rarely edit the titles of threads I've started, but I know that people who sell things on the swap meet part of the board use the feature a lot to let people know when something has been sold, put on hold, or re-listed. I think the inability to edit thread title will be frustrating to those folks.