On the web
Go here: https://apps.apple.com/account/subscriptions > tap on 1SE > Cancel Subscription.
On your iPhone's Settings
Go into your iPhone's Settings > [your name] > Subscriptions > 1SE > Cancel Subscription.
In the app
Tap Profile on the tab bar > gear icon at the right > Manage Subscription > Cancel Subscription.
Canceling a subscription does not refund previous payments; it only prevents renewal. For refunds on the remaining subscription time, please contact Apple.