If this is the first reconciliation made on a bank account, you can follow these steps on how to edit an incorrect opening balance.
Go to Settings ⚙️, then select Chart of Accounts.
Locate the account, then go to the Action column and select Account history.
Find the opening balance entry. Tip: If you've got the Date column sorted from newest to oldest, the opening balance entry may be at the end of the list.
Select the opening balance entry.
Edit the amount. Tip: If you're not able to edit the amount directly on this screen, in some cases you can edit by selecting the Edit ✎ icon.
Select Save.
Please note that once you edit or delete a transaction you've already reconciled it will affect the opening balance on your next bank reconciliation and you'll get a message on the Reconcile page. Furthermore, if you would like to undo entire reconciliation on a specific period, you can reach out to your accountant. Only accountants can undo entire reconciliations.
For more detailed information, you can check on these articles: