I'm here to help how you can undo a reconciliation. Feel free to follow the steps I'll laid out below.
You can only unreconcile one transaction at a time. If you need to start over, I'd suggest reaching out to your accountant. They can undo the entire period at once using QuickBooks Online Accountant.
If you prefer to do it by yourself, follow these instructions to undo reconciled transactions one at a time:
From the Accounting menu, select Chart of Accounts.
Find the account and click View register.
Review the check column. If the transaction is reconciled, you’ll see an “R.”
Select the box and keep clicking it until it's blank. It removes the transaction from the reconciliation.