All subscriptions automatically renew whether it is the monthly, quarterly, or yearly plan. However, you are able to cancel and/or modify your subscription at any time directly through your account's Settings page. For more information on how to stop payments, please see here: How do I cancel my subscription?
As all subscriptions fees are billed at the start of your subscription period, if there is any time remaining on your subscription should you decide to cancel, you will still have all the features of an upgraded account for the remaining time you paid for.
For example: If you sign up on January 4, your next billing date will be February 4. If you cancel before this renewal date, your card will not be charged again but you will still have access to an upgraded account until February 4.