There are two ways to edit a thread title, one from within the thread itself, and one from the forum page that the thread is on.

To edit from within the thread:

Click the edit button in the first post then select go advanced, then you'll see that you the option to edit the title.

To edit from the forum page:

http://www.menewsha.com/img/24237/64...881b3f5ec3.jpg

Double click next to your thread title (not on it or you'll just end up going into your thread) and the title will turn into an edit box. Make your changes and then hit enter.