Do you mean the thread title? If so there are options available for thread creators by which you can edit your thread. Posts don't have titles. Hope this helps.
(Never started a thread with my iPhone so I'm not to sure how it works there)
You can't do this from directly within the app. However, if you tap the smaller thread title at the top of the thread view within the app, you'll be taken to the "full" version of the page where you can rename a thread as you normally would.
