ADMIN: System → Manage Currency Rates
The System → Manage Currency Rates option contains a table of Base Currencies and Allowed Currencies, with the relative value of each compared to the Base Currency. Each Base Currency will have its own row, and each Allowed Currency will have its own column. When the row value and column value for a cell are the same (for example, where the US Dollars Base Currency and Allowed Currency intersect), the rate will be 1.0000. All other rates in the same row will be a ratio of this.
Allowed currencies and the default currency must be set separately via System → Configuration → Currency Setup tab → Currency Options panel.
If you have set up Magento to automatically update currency rates in the System → Configuration → Currency Setup tab → Scheduled Import Settings panel, you do not need to manually set the values here.
You can enter the exchange rates of the other currencies manually, or import them from an external currency rate provider in the Import Service drop-down. By default, Magento is configured to import rates from Webservicex. Select this from the drop-down, and click Import. The Allowed Currencies will populate with the most current conversion rates. If you had rates previously entered, they will now display underneath the field, as the Old rate. You can revert to these old rates by clicking Reset. When your currency rates are configured correctly, click Save Currency Rates.