I'll share some information about undoing a reconciliation, south.
The Undo option for a reconciled period is only available in QuickBooks Online Accountant. If you need to completely start over, I'd recommend inviting your accountant to your company and let him undo the reconciliation period at once. I'll guide you how:
Click the Gear icon
Select Manage Users.
Go to the Accounting firms tab, then click the Invite button.