AppleCare - Can I Cancel The Monthly Plan After Doing a Repair?
Most products are covered by AppleCare+ these days, so you would need to read the terms and conditions for your specific region where you purchased it to be sure.

https://www.apple.com/legal/sales-support/applecare/applecareplus/

In the US you can cancel the monthly iPhone (and most other iOS/watchOS product) plan at any time for any reason.

8.2 Monthly Pay Plans
You may cancel this Plan at any time for any reason.

It then spells out the details on how to cancel and what happens when you do.