Need to change my Visa card details and I don't know how?

On your iPad tap on your id in Settings > Store (Settings > iTunes & App Store on iOS 6+) and select 'View Apple ID' on the popup - that should give you a payments link on your account's page.

If doing it on your computer's iTunes then you should be able to edit your payment info by going into the Store > View Account menu option and logging into your account, and on your account's details page there should also be a payment link.

Changing payment info : http://support.apple.com/kb/HT1918