You may cancel at any time during your billing cycle for free. Any overage will be tallied and charged at the next renewal date, unless the billing profile is deleted, in which case overage is tallied and charged immediately.
To switch from a paid plan to a free plan you can either delete the site you're paying for, or reduce site's usage below the free plan limits.
You can request a refund of an annual plan within 15 days of purchase. Refunds are issued to the card that was originally charged, and may take up to 10 business days to complete.