yes, you can, I did it on launch night on the app, there was an area in blue that said upgrade here that you click it then takes you to next
I would imagine it would be easier on apple.com but that was not an option on 9.12 as site was down - perhaps clear your web history, do a good o'l shut down + restart - perhaps that will help but plenty of people here did it so if not on Mac then do on apple app
Did you have a balance? We have a balance, but we paid the "upgrade" fee - as long as we turn our device in?