Auto-renewal is turned on for all subscriptions by default, but it can easily be managed from your online account.
There are 3 different ways to access the cancellation/renewal options.
1) You can reach the Renewal Options through the My Subscriptions main page by clicking on the edit (pencil) icon next to your plan (shown in the image below).
That will take you to your plan options and you'll find the Renewal section on this page (shown in the image below).
2) This same Renewal section can be found in the My Account tab as well (with your other billing options).
Click on the turn off link and follow the instructions to complete your cancellation.
3) Alternatively, you can use this direct link to the cancellation page.
Turning auto-renewal back on
These same sections will display a different message if you auto-renewal is already turned off.
If you have auto-renewal turned off, you can click the button in the Renewal section to turn your auto-renewal back on and prevent your plan from expiring.