To set the currency that you want to be used on Prima, go to Administration > System Keys > Financial System Keys where you can then choose your base currency.
If you want to add a new currency to Prima, you can do this by going to General > Currency > Add New Currency.
If you need to change currencies you have set up already, you can click into it where you can set buying/selling rate decimal against base currency. You can also set a date and time that you would like the New Selling Rate to become active.