Any of these should work. On your iPhone is probably easiest, as that is automatically signed in.
iPhone: Settings﹥[You]﹥Subscriptions﹥[Select an active subscription]﹥Click the red Cancel link﹥Confirm the cancelation.
If you already canceled, then the Cancel link wouldn’t be available anymore. Instead you would see re-subscribe options.
If you already canceled, then the renewal date/next billing date would be replaced with an expire date.
- On any of your Apple devices: How to cancel a subscription from Apple - Apple Support
- On the web: How to cancel Apple TV+ - Apple Support
- On third party devices with the Apple TV app: Manage subscriptions in the Apple TV app - Apple Support
- On a PC with iTunes: Use a Windows PC to cancel subscriptions from Apple - Apple Support