How do I update my card details to purchase iCloud storage space

On an iPhone go to your setting and select your name with the picture icon

then press payment and shipping

add payment method

save and set to default payment

Go back and press iCloud

Press manage storage

Change storage plan

mohammed2138 Said:

"How do I update my card details to purchase iCloud storage space ?"

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Updating Card Details:

Money is deducted from your Apple Account first. Once zeroed-out, it is deducted from your card.