I'll ensure to provide you with the information about deleting a reconciliation in QuickBooks Online (QBO).
Currently, there isn’t an option to delete a reconciliation in QuickBooks. However, you can manually undo each transaction from reconciliation.
Here’s how:
Go to the Gear icon and select Chart of Accounts.
Select the account that has the reconciled transaction. Then, View register.
Locate the transaction to edit.
In the reconcile status column (indicated by a checkmark), repeatedly click on the box until it changes from "R" to blank.
Then select Save.
Moreover, if you want to undo an entire reconciliation, you can seek help from your accountant. If you already have an accountant added to your account, they can undo your reconciliation.
I've added here an article for future reference in reconciling an account, especially when you encounter reconciliation issues: Learn the reconcile workflow in QuickBooks.
Please come back if you have follow-up queries. I'm always here to help.