Foreign currency accounts can't be edited directly in the account register, so you need to edit the opening balance transaction itself to change a foreign account's opening balance.
Go to the Lists menu and click Chart of Accounts.
Double-click the foreign account whose opening balance you want to change.
Find the opening balance transaction and double-click that entry to open the transaction.
The opening balance transaction is usually the first entry in the register and is typically recorded against your opening balance equity account.
Make the necessary changes to the opening balance transaction and then save the transaction.
If you receive a message about the transaction being reconciled, click Yes to record your changes. QuickBooks doesn't change the cleared status of the transaction when the opening balance is changed.