To change the payment method from au payment :
Cancel your account first. After your account is moved to the Canceled state, restart your account with the payment method you want to use.
*You cannot change to iTunes Store Payment. iTunes Store payment can be selected when you sign up from iPhone/iPad/iPod touch/Apple TV or restarting the account signed up with iTunes Store payment.
*To change to Amazon In-app Purchasing, you need to restart on Amazon Fire TV.
*There may be some days left until the service stops depending on your billing cycle. In this case, please restart your account after the service stops.