Hi,
The exchange rate is relative to the main currency you have set in your configuration. E.g. If you have USD set and you want to add GBP and the exchange rate for it, you should find "US$1.00 to GBP". You can type "US$1 to GBP" in Google search for the currency conversion or use the one you prefer best. From Google the exchange rate for this is "1 USD equal 0.63 GBP". (see attachment below)
Now that we have the exchange rate. Go to "Currencies" and find the "British Pound" for GBP. Change "Rate" from 1.00000 to 0.63 and then save it and then Publish and Display it.
Once this is done, when you use the Currency Switcher to change from USD to GBP it will convert the prices to rates you set.