How do I cancel an Apple Care account for a device that I traded in and was removed from my account? I am still getting charged for it.

I initially purchased an Apple Watch 3, and then used it to trade in and receive a credit for a 7. I had initially set the apple care plan to be charged monthly. Is there a way to stop these charges? Since the device was traded in, it doesn't appear on my account or anything related to it. The subscription doesn't appear in my subscription area either.