I did that journey already, it’s very good in 2nd class. Difference between 1st and 2nd class on trains is usually not as big as on planes. Railjets are very comfortable.
To answer your question, I don’t seem to find a specific class upgrade ticket. Usually you pay the price difference between 2nd and 1st class with a member of staff, preferably before boarding (at the ticket office).
Or you buy a full fare 1st class ticket for that journey, or you buy a full 1st class Eurail/Interrail. When are you travelling? With a bit of luck, you can still score a saver fare in 1st class.
If you want to know what to expect on the Railjet: https://www.seat61.com/trains-and-routes/railjet.htm#RailJet_journey_like_from_Zurich_to_Innsbruck,_through_the_Arlberg_Pass