I have been paying iCloud storage for a long time and want to cancel because I have enough storage left my iCloud.
How can I cancel my iCloud payment?
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See here - Downgrade or cancel your iCloud+ plan - Apple Support
However, just so you know, when you cancel it you'll lose access to all the storage you've been paying for. You will be left with only the 5GB free iCloud plan! If you have any data in iCloud, it would be wise to remove it before canceling.