We can change the opening balance through the Chart of Accounts page. Let me guide you how:
Just a heads up, if the transaction has been reconciled, it can put your account to be off balance. That being said, I'd recommend reaching out to your accountant so you'll be guided.
Then, here's how you can change the balance:
From the Accounting tab, select Chart of Accounts.
Locate the account and then click Account history under ACTION.
Click the opening balance and then enter the new balance under the DEPOSIT column.