If its on PC, You can press SHIFT + TAB to go into the steam community in game, Then go to SETTINGS and in the FRIENDS tab, you change your nickname.
Alternativly, you can go to Steam outside of the game, and in the FILE dropdown, click SETTINGS, Then FRIENDS tab and change your nickname.
If you place a ^0-8 (only one number) it will change the colours of your name [PC ONLY]
If it is on PS3 or XBOX 360, you cannot change a ID once it has been set without creating a new account.
DONT FORGET TO THANK!