In the upper right corner, looks for the Settings link. (http://www.thefret.net/usercp.php)
In your Settings screen, look for Edit Signature about halfway down the left column. (http://www.thefret.net/profile.php?do=editsignature)
If you want to change the title above your avatar from "Regular Fretter", from the Settings page, go to Edit Profile (http://www.thefret.net/profile.php?do=editprofile).
Cheers!