When you have added a Subscription via the Mollie API or a basic subscription via the RCUR dashboard you can easily cancel the active subscription. To do this, lookup the customer in your RCUR dashboard and cancel the subscription via the actions.

Click behind the advanced subscription on the magnifying glass icon, then click in the options menu at the top right on "Cancel subscription". Now you have an option to select a date to cancel the subscription on.

Are you using recurring orders? Then go to your orders overview, choose for "Active subscriptions" and open the one that you want to cancel. Here you can update the subscription status to "Cancelled".