Go to the main menu(☰), select your name/email at the bottom, and select Manage Membership on the next screen.
Or
Select the ❤️ button and there will be a Manage Membership button under membership.
For users who purchased through Apple, the easiest way is to:
- On your device, go to Device Settings
- Select your name next to the Apple iCloud account.
- Select Subscriptions
- Select Watch Duty
- Select Cancel Subscription