Please follow below procedure to update opening balance of a particular account.
Go to Accounting->Chart of accounts, filter for the particular account, click on this icon 
present in action column tool tip of which displays “Edit account details”.
Click of which a pop-up box will open fill in details accordingly and click on “Update Account”.
Please note that the opening balances need to be updated for each accounting period. You need to ensure you are in the correct accounting period before updating the opening balance.