Hi again, jbirston, thanks for providing the screenshot. Once you enter the account balance in the Account detail window, you have to get in the account history to locate and edit that if needed. The fact that no balance amount was included in the account detail window before (at the initial set up of the account) would explain why you don't see any transaction with "Opening Balance" associated with it. Some users prefer to enter the opening balance as a transaction type. If you're unsure of which transaction in the account history corresponds to what you want to edit, I encourage you to reach out to an accountant to resolve this situation. They may either do an adjusting journal entry or help you in any other way to restore accuracy in your books.
Feel free to refer to my previous post for useful links and steps. I'm here if you have any other questions.