Hello there, @Sophie12.
Let me share some information about undoing reconciliation in QuickBooks Online.
The Undo option for a reconciled period is only available in QuickBooks Online Accountant. I'd recommend contacting your accountant and let them undo the reconciliation period at once.
Otherwise, you'll have to manually edit each transaction to undo the reconciliation.
You can use the screenshots below for visual reference,
You can use this article for reference: How to Unreconcile a Transaction or Undo a Reconciliation.
That's it! Let me know if you have other questions about the reconciliation process in QuickBooks Online. I’m always here to answer your inquiries.