A 1SE subscription bought on iOS is canceled through Apple, either on the web or in your iPhone's Settings.
On the web
Go to apps.apple.com/account/subscriptions, then select 1SE > Cancel Subscription.
In your iPhone's Settings
Go into your iPhone's Settings > [your name] > Subscriptions > 1SE > Cancel Subscription.
Canceling stops the next renewal but doesn't refund past payments (see Apple’s guide to subscriptions). For a refund, contact Apple Support.