can't. Only a site admin can do it, but if it's changed, all your posts from the first user name will only be connected to that user name. So from now on, when you're searching your past posts & threads, they will only work for your new user name.
Another option would be to create a new user name & just login with that account.
But if you're dead set on changing your user name, send Jman423
a PM & ask him. He has the power...