Oh, all you have to do is to ask your Alexa device to cancel your subscription, then confirm the request on the Alexa app with which you set up the device. Then log into your Amazon account in a browser or the Amazon app, go to your services and subscriptions area, find the subscription concerned, and fill out the cancellation form, then confirm with the link sent to your email. Simple, really. Of course, if you don't have the Alexa app because what's the point of keeping around an app for managing something that's independent of your phone, who knows.
For the avoidance of doubt, I made all this up. I don't have the hardware, so I can't get you real instructions. I just imagine it will be more complicated than it should be.