Go to the Gear icon, choose Accounting and Settings.
Click on the Billing and Subscription tab.
Pick Downgrade next to your subscription plan details.
If the Downgrade link is unavailable on the Billing and Subscription tab, you can go to the Cancel button, and you'll have the option to downgrade there. For additional information, you can refer to this helpful article: Downgrade your QuickBooks Online plan.
Feel free to comment below if you have any more questions. I'm always here to help. Have a safe and productive weekend ahead!