I've got the steps you'll need on how to modify the opening balance, rsj5020.
You can modify the Opening balance via the Account history for the account. Here's how to do edit an incorrect opening balance:
Go to Settings and select Chart of Accounts.
Locate the account you want to edit.
In the Action column, click on the 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.
Here are opening balance-related articles for additional guide and reference: