1) Open admin panel and go to Localization >> Currencies Here you can edit any of the existing currencies and add new one.
2) Click add new currency and fill in all the fields.
Title of your new currency, code, like USD or EUR, symbol that will be shown before or after the value and the value.
3) Press insert button and you will get it in the lest on the left. Check your website and test the way your new currency works.
Feel free to check the detailed video tutorial below: