I understand the importance of undoing bank reconciliation in QuickBooks Online (QBO), Anabanaba. I'm here to share insights about this. 

As mentioned by my colleague above, the only way to undo the entire reconciliation would be through an accountant. As a workaround, you can manually reconcile the transaction from the bank register to ensure the accuracy of your books. 

Here's how:

1. Go to the Bookkeeping menu and select Chart of Accounts (COA).
2. Locate the account you previously reconciled and click View Register.
3. Locate the transactions and select them to expand the view.
4. Review the check column. If the transaction is reconciled, you’ll see an “R.”
5. Select the box and keep clicking it until it's blank. It removes the transaction from the reconciliation.
6. Click Save. Then close your register.

I've added here an article that will help you in case you encounter reconciliation issues: Fix issues when you're reconciling accounts.
 

Please come back if you need any further assistance with this process. The Community is always here to help.