Understood. I didn't buy it just for steam. Though I am interested in upgrading to the premium version but I don't see an option to do that other than paying full price for premium.
If you bought the regular version and you want to upgrade to premium, I think there are a couple of ways. One other user in the comments bought premium this way. If you want, you can return the regular version by telling support and get it as premium again. If they don't refund your money, I can email you the game in exchange for the money you paid for the premium version.