In QuickBooks Online (QBO), you'll have to manually undo a reconciliation via the register. I'll guide you how.
Follow these steps:
On the left panel, go to the Accounting tab.
Select Chart of Accounts.
Choose the account that you want to unreconcile.
Find the transaction that needs to be unreconciled, one at a time.
Click on the R in the top line of the transaction, which is in between Deposit and Balance. Clicking in this field will allow you to change the status of the transaction to the desired status. C = cleared R = reconciled
blank = none of the above
Though, you can also reach out to your accountant to help you with the reconciliation. QuickBooks Online Accountant has the ability to undo multiple reconciliations without manually editing individual transactions from within the register.
You can also print or export your reconciliation report once you're done. This will be your reference for the next month’s reconciliation.
Feel free to post any concerns you might have. I'm glad to help. Have a nice day!