You may cancel a subscription anytime via the subscription settings through your iTunes account. This must be done 24 hours before the end of the subscription period to avoid being charged.
You can view, manage or cancel your subscription in the following way:
1) Go to Settings > [your name] > iTunes & App Store.
2) Tap your Apple ID at the top of the screen, then tap View Apple ID.
3) Scroll to Subscriptions, then tap it.
4) Scroll to Subscriptions, then tap it.
5) Use the options to manage your subscription.
Info is also here: