As far as I know, you can't change a thread title, only a post title. I just tried on one of my old threads. It may be possible if you catch the mistake right away, like before any change shows up as an edit below the post, but I think once the ink dries the THREAD title cannot be altered.
If you want to change the THREAD title, contact the mods and explain why. If it is to fix an incorrect date of a tournament announcement, they may not have a problem doing it for you.
Your other option is to simply start a new thread with the corrected date in the title, and then just bump the new thread as needed. In your post you might want to mention that you have re-posted because there was a date error in the previous thread title so the mods don't think you are spamming the board. Just let the old thread slide into oblivion in the back pages. If you want (just in case someone might come across the old thread and post into it) edit your OP or add a post in the original thread to say there was a mistake in the date so you started an new thread - and provide a link. If no one has posted yet, edit the OP because if you add a post it will just bump the original thread back to the top, which you don't want.