To do this, you need moderation over the forum that you want to change this at.
To get moderation, post an administrive request over the forums with approve from your faction leader, only then you can do this.
First you need to have moderation over that part of the forums, which you can make an administrative request. Then on the page on the right corner you should see something called ''Thread Tools''. Click that then you'll be able to change the prefix of the thread.