Source: StarCraft II Blog ThingFree Name Change Now Available
by Blizzard Entertainment // Nov 2, 2010 10:58:19 PM UTC 245
All StarCraft II accounts have been granted one free name change. If you'd like to change your character name, log in to Account Management and choose your StarCraft II: Wings of Liberty game license. On your game license screen a "Character Name Change" button at the bottom will initiate the request. After confirming that you'd like to change your character name, simply log in to StarCraft II and you'll be prompted to select a new one. Please be sure to use this name change wisely.
For anyone who may have made a mistake or have since regretted their original name decision, you now have a one-time free fix! Who knows how much this will cost in the future!
Just remember to update your name should you make any changes to the StarCraft II player list!