In you dashboard, go to Billing > Pricing Plan.
At the bottom of that page (you have to scroll down) you'll find a big banner saying:
"Want to cancel your subscription?
Your account will become a Free Plan again."
Just click on "Cancel Subscription Plan" and your subscription will be canceled.
You will be able to keep using the paid tools until the end of the subscription period, but there won't be any further renewals.
If you notice any strange behavior after that, such as being charged incorrectly, send an email to firstname.lastname@example.org and we'll provide you with a refund!