Go to Billing → Upgrade and choose a lower plan (or Free). Unlike upgrading, a downgrade is never immediate — it's scheduled to take effect on your next renewal date, and you keep your current plan's full access until then. There's no charge for downgrading.
Changed your mind before the renewal happens? You can cancel a pending downgrade from the Billing page at any time before it takes effect.
To fully cancel and stop future billing (rather than downgrade to Free), use the Manage Subscription link on the Billing page, which opens your provider's billing portal.