To change your Avatar's physical appearance (height, weight, hair and skin color, etc.) you need to go to the Appearance section of your Profile. If you have purchased additional hair styles, you may select the one you want in the Appearance section as well.
Select the User tab at the top of the Vegas World screen (where your name is.)
From the drop-down menu that appears, select "Appearance"
Once you've got the Appearance section open, you'll have all sorts of choices for your Avatar.
All changes made in your Appearance Profile are saved automatically.