Let me share the detailed step-by-step guide to undo reconciliation in QuickBooks Online (QBO).
If you prefer to undo an entire reconciliation at once, you can reach out to your accountant with an active QuickBooks Online Accountant subscription. They have a unique reconciliation tool that aids in editing or undoing several transactions on a past reconciliation.
However, you can also perform it yourself by unreconciling one transaction at a time. Please follow these steps:
Go to the Gear icon and select Chart of Accounts.
Look for the account you want to reconcile and select View register.
To narrow the transactions to last year, click the Funnel icon dropdown and select Last year under the Date section. Then, click Apply.
Review the check (✓) column. A transaction is reconciled if you see an R symbol.
Click the box and keep clicking it until it's blank. Doing so removes the transaction from the reconciliation.
Select Save and close your register.
I've also added these articles that will help you fix issues when you encounter errors during reconciliation: