You can only manually undo each reconciled transaction one at a time, Les.

Here’s how:

1. Go to the Gear icon, then Chart of Accounts.
2. Select the account that has the reconciled transaction.
3. Click View register.
4. Locate the transaction to edit.
5. In the checkmark column, repeatedly click on the box until it changes from "R" to blank.
6. Hit Save.
   

For more details, visit this material: Undo or remove transactions from reconciliations in QuickBooks Online.

You can also invite your accountant, as they have the option to undo the entire reconciliation at once. They can follow this article for the steps: Undo a client’s reconciliation in QuickBooks Online Accountant.

Feel free to revisit this thread if you have any follow-up questions about reconciliation.