Open Settings → Plan and billing. The Current plan card shows your plan and when it renews.
Choose to upgrade, switch plans, or cancel. Payments are handled by Stripe.
Cancelling never deletes anything: your account moves to Free's limits, and everything stays readable and downloadable.
Subscribed in the App Store or Google Play? Change or cancel it in that store's subscription settings. In a company workspace, the workspace's owner and admins manage billing, and your own account has nothing to buy.