You must manually change each transaction before you can Undo Bank Reconciliation in Quickbooks Online. To do so, check your bank statement and/or reconciliation report to see which transactions need to be reconciled. After you've completed that, you can undo your reconciliation.
Select reconcile from the gear icon at the top.
Click on history by account at the top of the screen to go to the history by account page.
Select the report period by clicking on the account you want to change. The required account can be found by glancing at the statement's ending date.
Select undo from the drop-down list in the action column.
A confirmation window will appear, which you should examine before clicking yes to proceed.
Click the "OK" button.
Note that if you undo reconciliation, all subsequent reconciliations will be undone as well.