You can Undo an entirereconciliation if you're using the Accountant version of QuickBooks Online (QBO) or reach out to your account to do it for you. However, you'll have to manually undo a reconciliation via the register if you have the regular version.
Click the Gear icon, then select Chart of Accounts.
Find the appropriate bank, then click View register.
Select the transaction to edit.
Under the Check column, click the box repeatedly to change the status to "Blank". ("C" - Cleared, "R" - Reconciled, and "Blank" - Not cleared or reconciled)