You can downgrade your account at any time. Just go to the Subscription section by visiting Account › Subscription › Change plan.
If you downgrade from a paid plan to the free plan, you will maintain access to the features of your paid plan until your next billing cycle would be due to start, at which point you will automatically be downgraded to the free plan.
If you change your mind after scheduling your subscription's cancellation, you can simply reactivate the subscription and you will be billed as usual.
If you would like a refund, you must request one within the first 14 days of your subscription start date.