I got your back in this query about undoing a reconciliation in QuickBooks Online (QBO).
You have two options if you want to undo a reconciliation period. The first is you do it by hand, which is a readily available option.
I'll show you how:
Highlight the Accounting tab.
Select Chart of Accounts.
Locate the bank account in question, then tap the View register link.
Look for the transactions you want to undo its reconciliation.
Under the column with a Checkmark, click it repeatedly until R changes to blank.
When done, hit Save.
Refer to the screenshot below for your reference:
The second option is available if you have a QuickBooks Online Accountant user that's in your company, also as an accountant. If that's the case, they can undo the whole reconciliation period in only a few clicks of the mouse. You can forward this article to him as a reference: Undo a client’s reconciliation in QuickBooks Online Accountant.