A bank reconciliation can be undone in two different ways. Let me discuss them with you.
In the bank register, you can manually undo a reconciliation of each of your affected transactions.
On the Banking page, click Go to Register.
Select any transaction.
Under the Check column, click until the box becomes empty.
Click on Save.
You'll have to repeat these steps to every transaction to undo their reconciliation.
Your second option is to seek help from your accountant. If they can access your QuickBooks Online (QBO) account through their QBO for Accountants account, they can undo your old bank reconciliations in one go.