You have to go to the “Account & Settings” page.
Select the “Your account” tab on amazon.com/mytv.
If you receive access to Prime video as a part of the Prime video-only subscription.
After this, tap on the “End membership” on the page and confirm.
Alternatively, if you have access to the Prime video membership subscription then select the “Edit on Amazon in the “Your membership” section and select the “Cancel” from the resulting page.
Point to remember: You can always reach customer support if you have confusion related to how to cancel prime subscription.