As already mentioned by Jim, you can only change/edit the thread title/contents within 30 mins of posting it. After that, it's a mod/admin only thing.
One thing to note here is that if you for some reason need the thread title to be changed or the content edited a bit (due to typos etc.), you can always "Flag bad post" the first post of that thread and request the moderator/admin to do it for you. This is assuming your request is is accordance with the rules.