How to cancel a subscription that already renewed

Managing auto-renewing subscriptions -

http://support.apple.com/kb/HT4098

If you want a refund, speak directly to Apple Support;

Get Support -

https://getsupport.apple.com/

M1 Mac mini, macOS 12.0.1; Watch, series 5 GPS/cellular, watchOS 8.1; iPad Pro 2nd Gen, iPadOS 15.1; iPad Air 2, iPadOS 15.1; iPhone 12 Pro, iOS 15.1; TV 4th Gen, tvOS 15.1; HomePods, audioOS 15.1 and Airport Express w/ Airplay 2

Go to https://reportaproblem.apple.com/?s=6 and select 'Request a refund' from the menu in the page. You will need to ask them to cancel the subscription as well as this isn't provided as an option (if you cancel it yourself it will continue until the next renewal). Refunds are at Apple's discretion. Note that the refund, if granted, can take up to 30 days to be paid into the payment method used for the purchase - please see https://support.apple.com/HT210904