How do I edit a thread?

I made a thread a while back offering services, how do I consistently edit this thread? I want to update prices(since we do promo) and update the vouches section.

here is a link to the post:

http://www.mpgh.net/forum/showthread.php?t=783443

Edit: I can edit this thread, just not that one. The only differences are its older and it got closed at one point then re-opened. Any help staff?

Thanks,

Silentrath