How can I change my payment method on iTunes?
My in app purchases are being declined, how do I change the payment method?
iPad (4th gen) Wi-Fi + Cellular, I don't know the OPERATING SYSTEM
If you tap on your id in Settings > iTunes & App Stores and then tap on 'View Apple ID' on the popup you should be able to log into your account, and on the screen that you are taken to there should be a link for Payment Information.
If using a credit card then it needs to be registered to exactly the same name and address as you have on your iTunes account. If you want to use a debit card then I'm not sure that they are still accepted, there have been a number of posts recently about them being declined.
When trying to buy the in-app purchases if you are getting a message to contact iTunes Support then you can do so via this page : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption
(In terms of the iPad's iOS version from your tagline, your current version is shown on the iPad in Settings > General > About > Version.)