The ability to Undo a reconciled account is only available in the QuickBooks Online for Accountant version. Here's how:
Go to the Gear icon.
Pick Reconcile from the Tools column.
On the Reconcile an account page, select History by account.
On the History by account page, click the Account and Report period to locate the reconciliation to undo.
Choose Undo on the Action column, then Yes.
In the final confirmation window, pick Undo and OK.
However, if you're not seeing the Undo option, it's possible that you're using the regular QuickBooks Online version. If that's the case, you need to manually edit each transaction to undo the reconciliation. Here's how:
Go back to the Gear icon.
Pick Chart of Accounts.
Locate the appropriate account for the transaction.
From the Action column, select View Register.
Identify the transaction to edit.
In the reconcile status column (indicated by a checkmark), repeatedly click the top line of the transaction to change the status.