If the board would allow me to post an image/screenshot, I would show you. But since it won't, here's a brief description. This works if you are on a computer - not sure about it if you're using your phone via the mobile site.
You can only edit a title on a thread you create. Here's how:
1. Open the thread
2. Look across the screen to the right
3. Look for the gold button titled "Post New Thread".
4. Now look just left of that button to a link titled, "Unwatch Thread".
5. Now look just left of that link to a little wrench icon with a drop down arrow beside it.
6. Click on the wrench icon - revealing a "Thread Tools" menu.
7. The first option in that menu is "Edit Title"
