I'm glad to have you here in the Community, @Park2.
First,I recommend consulting with your accountant before doing any changes to avoid messing up your transactions. However, I can walk you through on how to manually undo bank reconciliation in your QuickBooks Online account. Here's how:
Go to the Accounting menu then choose the Chart of Accounts tab.
Look for the account and click on View register.
Select the transaction you wish to unreconcile.
Review the check column. You’ll see an “R" if the transaction is reconciled.
Click the box and keep clicking it until it's blank. This removes the transaction from the reconciliation.