Go to system preferences and type "short name" into the search box.
click the lock to make changes, change your short name to what you want it to be, and boom. 🙂
I would advise against this. It's very difficult to do, and you run the risk of borking your account completely. I think there might be a thread around here somewhere with instructions on how to accomplish it, but you're better off just making a new account and transferring everything over manually.
Go to system preferences and type "short name" into the search box.
click the lock to make changes, change your short name to what you want it to be, and boom. 🙂
They certainly didn't go out of their way to make it easy. For the Tiger version of this document, they even shortened it down to don't bother, create a new account and copy over files you want to keep 😀