You can update your payment method for your Apple TV + by following the steps below:

1. Open iTunes.
2. In the menu bar at the top of the iTunes window, tap or click Account, then tap or click View My Account.
3. Sign in with your Apple ID.
4. Make sure that what's listed on the Account Information page matches the information on file with your financial institution.
5. Tap or click Manage Payments to the right of Payment Information. (If you're using an older version of iTunes, tap or click Edit to the right of Payment Type.)
   
6. To update a payment method, tap or click Edit. To add a payment method, tap or click Add.
7. From here, you can update, remove, or add payment methods. If have have multiple payment methods, you can change the order. Apple will attempt to charge payment methods in order from top to bottom. You can't change the order in which Apple ID balance is charged.