You can change it easily on the profile menu.
1. Click the proflie on the left-upper side.
2. Click the pen icon for change name.
3. Click the Free button(at the first time)
4. Enter the name and re-enter the name with confirm and change.
Make sure that you choose your new name careful, as you can only change your name freely once. If you using the free chance you can change name with goldbar.