For publishing configuration. Run following command
php artisan vendor:publish
Once publish is completed, open config/currency.php. It contains two element. One is url which is an api endpoint and another one is baseCurrency. You can change baseCurrency and set standard currency of your own, however url value is not meant to be changed.
if baseCurrency is set to GBP, all the conversion rates will be show in respect to GBP as a base. Passing date will give the exchange rate for that particular date. And passing conversionCurrency will give the exchange Rate for that particular conversionCurrency in respect to the specified baseCurrency.
$exRate = Currency::exchangeRate('USD');
returns all available exrates with respect to the USD as a base currency for present date.