If you're on a monthly plan, your subscription expires at the end of each term unless you manually renew your Shell for another month via your console.
However, If you opt-in for autorenewal via console but now would like to cancel, please follow the following steps:
In your account console, you can cancel all subscriptions.
If you have multiple Shells™, click "Edit" on the Shell™ you'll like to cancel. On the next page, there will be a "Cancel Your Subscription" button in red.
After canceling a Shell, the Shell will remain active until the end of the term and no new charges will be made. Therefore, no further action will be required on your end.