I can't update my digital copy of Shovel Knight on the 3DS

So I got Shovel Knight on Saturday and have been loving every second of it and when I heard of the Plague of Shadows update I was ecstatic! So I went to the eShop and downloaded it but when the download finished it just gave me an error, I've looked in to it and the answer I got is that I have to have the cartridge if I want to update. Can you guys help at all? Thanks in advanced!