I want to change my payment info but it keeps saying my security code is invalid. What does that mean?

The security code is the 3 or 4 digit code on your card - on most cards it's on the back of the card on the signature strip, but on some cards (e.g. Amex cards) it's on the front.

Credit card security code : http://support.apple.com/kb/HT3541

I tried again and its still saying the same thing

Your credit card is registered to exactly the same name and address (including format and spacing etc) that you have on your iTunes account : http://support.apple.com/kb/TS1646 ?