You can cancel your iOS app subscription as follows:
Go to "Settings" on your iPhone. Then tap on "iTunes & App Store".
Now tap on your "Apple ID: " at the top of the screen and then on "Show Apple ID". You may need to sign in with your Apple ID password first.
Now tap on "Subscriptions"/"Subscriptions", select the subscription you want to manage and tap on "Cancel subscription". This will disable the automatic renewal at the end of your current subscription period.