It should be easy, really - just go to "User Details" and tap the box that says your current name to erase it and then type in your new one before clicking save. I'm sure you know what to do when you're there, but yeah. UNLESS you mean in the game, then no. There is no way to change your username in-game otherwise, you'll just have to make a new account I guess. The registration does open sometimes, a least two hours or so a day.