I bought Toren but didn't see that there was a deluxe edition. Can I just upgrade instead of spending another 12$?
More generally, why are different editions of a game not linked or even shown on the game's page like Steam?
I had to search just to find out there was a different edition. Look at Guild of Dungeoneering.
You have the dlcs on the main game's page, but you don't have the complete pack. Again, had to search to even find it.

If you haven't downloaded it yet, you can get a refund and simply buy the deluxe edition afterwards.
By the way, the only thing that seems to be different about the deluxe edition of Toren is that it includes the soundtrack as a downloadable extra.

Unfortunately, there is no option to upgrade Toren. Assuming you haven't downloaded any of the files, you could ask support to refund it for you: http://www.gog.com/support/contact/my_orders_and_payments (select order change as the problem type and explain the issue in the field below).

Downloaded, installed and played. That's why i asked about upgrading.

Well, that sucks. You could try asking support in the link I gave you above if they would be willing to provide you with a discount (equal to the amount that you paid) for the Deluxe Edition.

Not only there is no way to upgrade, but if you buy a Deluxe edition, GOG will not provide you with a key for the base game you already own (Please note that Toren is included in Toren Deluxe Edition but you already own this item, and will not receive another copy when you buy this pack).

2 possibilities

a) you wait for the title to be on sale (-75% or better), and that will compensate the prejudice
b) considering the "Deluxe edition" is just the soundtrack , you leave it s it is and you use your 12$ to get something else you want.

^^This. Ask support. They're usually pretty nice and forthcoming.