Here are the steps to cancel your magazine subscription on an Apple device:
Go to your 'Settings' app
Scroll and tap on 'iTunes & App Store',
Tap on your Apple ID, then tap on 'View Apple ID' and enter your password
Scroll down and tap on 'Subscriptions'. On this page, you should see all your app subscriptions - tap on the one you want to cancel.
Your current subscription details will be shown - turn auto-renewal to OFF.