Click the Gear icon ⚙ at the upper right corner and choose Account and Settings.
From the left panel, click the Billing and subscription tab.
In the QuickBooks section, select Cancel subscription or Cancel online.
Tap the Continue to cancel button (if it shows on your screen).
Follow the next steps. Tick the Submit and cancel subscription.
Hit Got it.
In case you're ready to reactivate your account, you can always visit this link for the detailed steps: How to resubscribe or reactivate QuickBooks Online. Also, this will provide you details on how long you can access your existing file after the cancellation date.
Let me know if you have any other QuickBooks concerns. I'd be more than willing to answer them for you. Have a great rest of your day!