I think you can edit your thread title within a certain time frame (like two minutes) after initially posting it, and the title change will be reflected in the forum. At least, that has been my experience. (Similarly, if you edit a post within a certain short time -- also like two minutes -- after first posting it, the "edited by" does not appear at the bottom of the post.)

But after that, you have to ask a moderator to make the title change for you. @:-)