HT1918: iTunes Store: Changing your payment informationLearn about iTunes Store: Changing your payment information
Currently Being ModeratedNov 21, 2012 5:12 AM (in response to elmallah)
If you know what it currently is then go to https://appleid.apple.com/, click 'Manage Your Apple ID' on the right-hand side of that page and log into your account. Then click on 'Password and Security' on the left-hand side, answer your security questions and you should be able to change it.
(You should also be able to do it by logging into your account on your computer's iTunes via the Store > View My Account menu option and clicking Edit to the right of your id at the top of the screen that you are taken to.)
If you can't remember what it currently is then on https://appleid.apple.com/, click 'Reset Your Password'