Enter your license key and the email it was purchased with. You'll be able to update billing, and see (or deactivate) the sites this license is active on.
Deactivating a site frees its slot immediately — the plugin there will just show as unlicensed until it's re-activated (or reactivate it right on that site if it's still yours).
Update your card, view past invoices, or cancel your subscription — all handled securely by Stripe.