If currently, you're using QuickBooks Online Accountant, use the Undo a reconciliation feature to unreconcile previous transactions. To visually guide you through the steps, click on this link: Undo a reconciliation.
However, if you’re using a different version, you can manually unreconcile each transaction to undo the reconciliation. Let me walk you through with the process.
Go to Accounting tab on the left panel.
Select Chart of Accounts.
Choose the specific account, then click on View Register.
Select the transactions you want to unreconcile.
Keep on clicking the R status until it shows blank or cleared (C).