Open My account → Subscription. At the top you see your current plan and the next billing date.
Changing plan
Choose the billing cycle with the Monthly / Yearly switch.
Click Select plan on the plan you want.
Upgrade: takes effect immediately.
Downgrade: takes effect at the end of the current billing period. Content exceeding the new limit is set to private – which content, you decide via Set order.
Cancelling
Click Cancel subscription. Cancellation takes effect at the end of the billing period. Your buyers keep access to content they have already purchased.