The method for canceling my iCloud backup is not working. The free option won’t highlight and Done won’t work
How do I cancel my iCloud backup subscription? Downgrading to FREE is not working and DONE does not work
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See the following from Downgrade or cancel your iCloud+ plan - Apple Support
Tap Done. If you can't tap Done, make sure that you're signed in with the same Apple ID that you use for your iCloud+ plan. You can also try following these steps on a different device. If you still need help, contact Apple Support.
Also see:
If you downgrade or cancel your iCloud+ plan, the change takes effect after your current subscription billing period ends.1