If you subscribed using an iPhone the following steps will help you. (from https://support.apple.com/en-au/HT202039 )
On your iPhone, iPad, or iPod touch
On the Home screen, tap Settings.
Tap iTunes & App Store.
Tap your Apple ID at the top of the screen.
Tap View Apple ID. You might need to sign in.
Under Subscriptions, tap Manage.
If you don't see a subscription in iTunes and are still being charged, make sure that you're signed in with the correct Apple ID. If so, you might have subscribed directly with the provider and not through iTunes. In this case, contact the content publisher or app developer directly to cancel your subscription.
Tap the subscription that you want to manage.
Use the options to manage your subscription. You can choose a different subscription period, or turn off Automatic Renewal.
Turn off Automatic Renewal to cancel a subscription. Your subscription will stop at the end of the current billing cycle.