1. A disturbing trend of bad advice has been happening as of late that instructs QBO users to go directly to the account register and manually change the status of transactions to "R" for reconciled, which means that an actual "Reconciliation" is not being performed and therefore cannot be "undone" without going back into the account register and manually removing the "R" from each of the transactions that were "manually" reconciled.
2. If the reconciliation was done correctly, you should be able to go "Reports"/"For my Accountant"/"Reconciliation Reports" and choose the reconciliation and select the "Action" to "Batch Undo" the reconciliation. This option is only available within QBOA (QuickBooks Online Accountant) firm accounts, and cannot be done if the actual reconciliation process was not performed in the first place as noted in item # 1.