In order to change or update your payment information, you will need to access your iTunes store account. You can do this by following the instructions below for both the iPhone and iPad:
1. On the home screen, select Settings.
2. Select iTunes & App Store.
3. Select your Apple ID (Please note that you might need to sign in using your Apple ID).
4. Select View Apple ID.
5. Select Payment Information.
6. Change your information and select Done.
Please note that after you change your payment information, the iTunes Store will place an authorization hold on your credit card. For more information regarding the authorization hold, please visit Apple's Support page: https://support.apple.com/en-us/HT201292