You can log into your account of your computer in iTunes
Launch iTunes, Click on iTunes Store on the left side. Click on - Account - under the Quick Links heading on the right side of iTunes. Enter your password. Under - Apple ID Summary - Click on - Edit - next to Payment type.
Or ....On the iPad
Go to Settings>Store>Tap on your ID. Tap View Apple ID. Enter Your Password. Tap on Payment Information in the pop up window.
You must go on to iTunes using a computer not an iPhone or iPod. If you had an form of gift card (other than the iTunes gift card itself) you can't change your information unless you actually put a legit credit card number in. I had this problem before where I was 15 and put a Visa gift card as a credit card on iTunes and then it wouldn't let me edit it. I bought $30 worth of music using the card and never got charged for it. Anyway, this message would pop up every time I wanted to download something saying I needed to update. Eventually I couldn't do anything on that account anymore so I made a new one. Making the new one was really hard in the beginning because I they have it set up so you have to enter a card number in the new account sign up area.
I got to make my new account without a credit card because what you have to do is
-go on to iTunes
-go to the store
-look to the right and find the "quick links" area.
-scroll down past all the top songs, albums, movies ect until you get to "free apps" (make sure its the free ones so it can actually work)
-without being logged in to any account hit download for a free app
-iTunes will ask you if you want to make a new account or sign in, choose new account
-once in new account you will be asked what email and password you want along with other security stuff.
-when you get to billing there will be the place for credit card information and then all the way at the end of the list of credit card brands will be the option labeled "none". Click it and fill out the rest of the form and get your new account without credit card information