User CP -> Your Profile -> Edit Your Details
Your Profile is the big block header in the left sidebar when you access your User CP. Just below it is a link that says
Edit Your Details. Click that.
You will see on the second block header which is
Optional Information. The first option is
Custom User-title. Type in what you want in the open dialog box. Then, scroll down to the bottom of the page and click
Save Changes. And you're all set.
If you're having trouble locating the page,
this is the page I'm telling you about.
Oh, and you can't delete an account. I don't believe the staff will do it for you, either, because there really is no need to delete it. >:< Ask them if you wish.
Edit: Beat. x_x