You’ll need to go into your iPhone's Settings > [your name] > iTunes & App Stores > Apple ID > View Apple ID > Subscriptions > 1SE > Cancel Subscription.

This is the only way Apple allows subscriptions to be canceled. 

FYI: Canceling doesn’t provide a full refund. It only stops renewal.
Please talk with Apple for a refund of the remaining time on a subscription.

Here’s a helpful Apple article on it:

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