Upgrade iCloud
How do I use my apple gift card to upgrade my iCloud
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When you redeem an Apple gift card it will be added to your Apple account balance.
To use this balance for your monthly iCloud+ subscription you will need to change your payment method - as described in Change, add, or remove Apple ID payment methods. Follow the example to drag Apple Account to the top of the list of payment methods. This will then be used for your next purchase. Make sure though that these funds are still available on the due date of your monthly iCloud+ payment (or the first payment if you are upgrading from the free plan to one of the three paid plans).
After payment has been made using your Apple balance you will have to do one of two actions for the next monthly iCloud+ payment: add more funds to your Apple balance or change your payment method to a credit or debit card.