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I'd be glad to help you delete a reconciled transaction in QuickBooks. To get started, please know that the Undo option is available in QuickBooks Online Accountant only. You'll want to invite your accountant to your company, and let them undo the entire reconciliation period at once.
Although, you can also unreconciled the transactions in your Chart of Accounts one at a time. I'll show you how:
Select Accounting from the sidebar menu.
Go to Chart of Accounts.
Find the account holding the transaction, then click View register from the Action column.
Select the transaction you want to unreconciled to expand the view.
Review the check column. You’ll see an R if the transaction is reconciled.
Keep clicking the checkbox until it becomes blank to the transaction from the reconciliation.