You are able to cancel Prime Video subscriptions online.
Note: If your subscription was created through a third-party, such as a mobile service provider, contact them for further assistance.
To turn off Auto-Renew on primevideo.com:
If you receive access to Prime Video as part of a Prime Video-only subscription:
If you receive access to Prime Video as a part of an Amazon Prime membership:
Any additional subscriptions tied to your Amazon Prime or Prime Video membership will not renew once your Amazon Prime membership ends.