What you cannot never change: thread title
What you can change: post title and body
When you start a post, you fill in the "Subject" box. This becomes the thread title and post title. These two titles start off one and the same.
After you posted, you can go back and edit the post title and body as many times as you want -- but not the initial thread title.
The unchangeable tread title shows up on the branch's listing of threads. The changeable post title shows up in the specific post itself.