Instructions
Navigate to the Windows LIVE homepage (see the References section for a link).
Click the "Sign in" link in the upper-right-hand corner of the page.
Enter the email address and password associated with your Windows LIVE ID (this is the account you created when you signed up for Xbox LIVE). Sign into the account, which will bring you to your profile page.
Scroll to the bottom of the webpage and click "Account" in the lower right-hand corner.
Click "Registered Information."
Change your date of birth and click "Save."