Thank you for taking the time posting in the Community. I'd be happy to help share ways to undo a reconciliation.
You can manually unreconciled individual transactions in QuickBooks Online. You need to edit each transaction and change the reconcile status in your register.
Let me guide you through the steps:
On the left pane, select Accounting.
Select Chart of Accounts.
Locate the appropriate account of the transaction.
From the ACTION column, select View Register.
Identify the transaction to edit.
In the reconcile status column (indicated by a check mark), repeatedly select the top line of the transaction to change the status of the transaction. Codes indicate the status of the transaction (C- Cleared, R - Reconciled and Blank - Not cleared or reconciled).
These resources should help to get you on the right track to cancel the reconciliation in your account. Feel free to add a comment below if you have any other questions about this process. I'll be happy to be your guide once again. Have a great day ahead.