How to Change exchange rate type and exchnage rate value

Dear All,

Payable super user and after creating invoice in EUR, match it with PO, and pay it  in JUL; he discovered that exchange rate should be different. He need to change the exchange rate type to user in order to set the correct exchange rate.

Simply; how to set the exchange rate to the desired rate?

Thanks in advance.

Regards,

Shareef