How do I change my payment date for my iCloud storage ?

It is billed once a month based on the date you started the subscription. The only way to change the date would be downgrade to the free 5GB plan and then re-subscribe on the date you wish the bill to be issued.

Use gift cards to prepay the charges, or click here and ask the iTunes Store staff for assistance.

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Use the link I provided above. The staff at the physical Apple Stores aren't the same people.

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