You can cancel your plan at any time by going to Account Settings and then to Billing.
On the billing page you'll see an option to 'Update Plan' as shown below:
On the subscription plans page it'll show the plan you're currently on and you can click on the 'Cancel' button to end the subscription.
After you cancel your credit card will not be billed again.
You'll still have access to the app until the end of your current billing period.