You can cancel your subscription at any time by clicking the "cancel" button on your subscription settings page, here. If you're not seeing a cancellation button there, it could be that you've already canceled through another service, like the google play store, or PayPal. When you cancel through your payment service like this, that button will disappear.
It's worth noting that canceling your subscription will not result in a refund, it just prevents you from being charged again in the future. If you've forgotten to cancel your renewal and were recently charged, please follow the steps listed in our refund article.