If you didn’t want to subscribe for more than one month or year, you can cancel future billing from your Subscriptions page at any time and clicking on the link to “Cancel future payments”. You can do this immediately after subscribing, and still retain the remainder of the time that you paid for! After doing so, you'll receive an email confirmation that your subscription has successfully been canceled.
iOS: All subscriptions purchased through Apple must be cancelled through Apple. Head here and follow their instructions for cancellation.