1) If you're on Pro plan, go to your container and click on three dots at the top right.
Choose Cancel Subscription.
You will become a Free plan user when the current billing period ends.
2) If you're on Business plan (or higher), please write to support.
Your container will be downgraded to the plan you'd like. The sum of money will stay on your stape balance and will be used for the payment in the next billing period of your container.