Subscriptions on Apple devices are managed from within the Settings app;

Go to Settings > [your name] > iTunes & App Store.

Tap your Apple ID at the top of the screen.

Tap View Apple ID. You might need to sign in or use Touch ID.

Tap Subscriptions.

Tap the subscription that you want to manage - looking for Puzzles Plus subscription.

Use the options to manage your subscription. You can choose a different subscription period, or tap Cancel Subscription to cancel your subscription. If you cancel, your subscription will stop at the end of the current billing cycle.

For further information with instructions on how to do this from your iPhone, iPad, or a desktop computer, Apple have further instructions here: How To Manage Your Subscriptions