Are you getting an error when trying to cancel the subscription? It's sad to see you go and not continue your QuickBooks Online subscription. Here's how to cancel the account in product:
Log in to QBO using a web browser like Google Chrome or Safari.
Click on Settings or Cog wheel icon.
Click on Account & Settings.
Click on Billing & Subscription tab.
Click on Cancel subscription.
Follow the prompts to completely cancel the subscription.
Click on Continue to cancel.
I would recommend exporting your data and list for back up purposes. You'll have 1 year read only access on your data from the cancellation date. So be sure to export the dat and reports needed. To export data, click on the Settings icon, and click on Export data.