- Go to 'Customers' window from 'View' menu,
- Select a customer or Add a new customer,
- Click 'Edit' button on toolbar to edit the details of the selected customer,
- Go to 'Info' tab and edit the opening balance to the desired amount,
- Click 'OK'.
Similar can be done for a supplier.
Also the 'Opening balances' for many accounts can be changed from the 'Chart of Accounts' window.
I suggest you should check and verify 'Balance sheet' and 'Income statements' on modifying any opening balances.
Hope this helps!