Currencies are managed in the currency list, which can be accessed through "Admin" ⇒ "List Currencies".
To add a new Currency, click "New" in the top of the list and fill out the fields in the Currency form.
Don't forget to save and the new currency is added to the list.
You can modify existing currencies. To do so, just click on the name of a currency in that list and a form opens and you can modify the properties of this currency. Again don't forget to save your changes.
To remove one or more currencies from the list, either click on the "trash bin" icon in the last column of each row and confirm to remove the currency or select multiple currencies using the checkboxes and click on "Remove" in the toolbar.