You can Top Up your number in a few different ways. The easiest in is to Top up via Nova.is and select Top Up in English at the top of your screen. 

You can also Top Up your number via the Nova app. To Top Up you have to log in with your phone number and click on the first option available, 'Fyllt'ann'.

If your Nova app is set in Icelandic you can change the language easily. 
You can of course aways buy a Top Up in any Nova store as well as through the customer support via the online chat on nova.is