Simple answer. There are two main ways of doing this so I'll make it multiple choice.
- PM a forum administrator and ask for the change.
- Post your request in Rename Request and the first admin to spot it will take care of it.
Follow either of those methods and you will usually be renamed. Obviously there are certain requests that will not be granted, but we're pretty reasonable about it.
You said "if" so I'm not sure if you are merely curious, or actually want to be renamed. I'll let you follow through as stated above if you are serious.