Instructions
Open "StarCraft II" on your computer, then log in to your Battle.net account as you would normally to start playing.
Click your portrait in the upper right corner of the screen next to your name and achievement points, then click "Rewards" on the menu that appears in the screen below.
Click once on any of the illuminated characters you see to select that character for your profile. The changes take effect immediately. Any characters you cannot select will be greyed out and a lock icon will be displayed over on that character. These icons can be unlocked by completing various objectives in the game; mouse over a locked portrait to see the requirements for each one.