It's not possible to change currency for an existing opportunity or order.
You choose the currency in the Currency field when you create a new opportunity. The field is shown only when multi-currency is activated and more than one currency is active in your account.
To use another currency on an opportunity:
- Open the opportunity and click Create copy.
- In the copy, select the currency in the Currency field and save.
- Delete the original opportunity.
A copy of an order keeps the order's currency. To get an order in another currency, create a new opportunity in that currency. It keeps the currency when it becomes an order.