You aren't supposed to be able to change your birthday once you have entered it, but I've found a way. First go to your profile, and then click on edit profile. Now open an html editor (not Firebug, but stuff like Web Developer). Search for "UserBirthDay", and change the number to whatever you want. Then look for "UserBirthMonth" and change that to whatever you want. And finally, look for "UserBirthYear" and change that to whatever you want. Now, click "Save Profile"
WARNING
Don't set your birthday to a date that will make you younger than 14. You will get kicked out of your account and be forced to donate 30 cents.