Do you mean before or after the order is posted?
Before it is posted you can modify the exchange rate in the exchange rate table.
After the order is posted the only thing you can do is evaluate the accounts payable.
If you want to correct your earnings in the G/L you could do a manual transaction but that will not be reflected in your inventory so it will be a potential risk of discrepancy in the valuation of your inventory. So i would really not recommend it. Then it is better to create a credit memom and then repost the invoice with the correct exchange rate.