I want to change my iCloud storage subscription from monthly to annually

The fee will be applied to your AppleID each month only, whether it’s using a credit card or being deducted from your AppleID balance. The annual plans were discontinues years ago now. There are only the monthly subscriptions now.

Hi,

Help here >  Upgrade or downgrade your iCloud storage - Apple Support

iCloud storage is available only on a monthly basis.

Buy enough iTunes Store gift cards to prepay a year's worth of charges, and redeem them just before one comes due.

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