How to correct Exchange Rate based on new order date?

Hi,

One of our users has created a sales order using incorrect order date by mistake. So the system has picked up the wrong exchange rate. After realizing the mistake, user has gone corrected the order date. However, system did not update the exchage rate based on the new order date. To make things worst, sales update has been already done for this order. How can we correct this in the system now?

Thanks and regards,

Husni Ameen