You can downgrade your QuickBooks Online and QuickBooks Online Payroll subscription through the settings page of your QuickBooks account. Let me walk you how:
Click on the Gear Icon.
Click on Account and Settings.
Click on Billing & Subscription.
You'll see your current subscription, and click on Upgrade/Downgrade your plan.
Once done, you'll see the changes in your payroll fee on your next subscription billing date.