If you are looking to complete invoices in another currency you will need to complete the rates here. You can enter a Buy rate and Sale rate. If you are going to back date invoices then you might need to consider the Advanced method as shown below, this will help keep all the rates and give more accurate results when running profit reports
Exchange Rates Basic
A Currency will only be "Live" once an Exchange Rate is set next to it. You can click the Bank Rates button at the top of the page to get rates from a bank website.
You can select to use 2 rates, one for Purchase and one for Sales or click "Use Single Exchange Rate" at the top of the page
Advanced
You can turn on the Advanced option and this will store the rates for more accurate reports, use the below and set a trigger point at the bottom of the screen