On the top of the page you can see your current username. Put your cursor over it and you should get a drop down thingy with a lot of options. Click on the personal details text, it will redirect you to another page. Go down on that page and look on the left side. Under settings you have things like personal details, signature etc. Furthest down on that list you can see User name change.
I think there is a limit to how often you can change your username. After you've changed it you can't change it before x time has passed. At least it was like that on the old server, but I have no idea how it is now when they've changed it. Either way, make sure you change it to something you really want to have, because once you've changed it you might not be able to change to something else in a while.