Hello Olah29
If you are traveling with a Half-Fare travelcard and a 2nd class Supersaver travelpass, you can purchase a day class upgrade for 1st class travel as well as discounted, route-related class upgrades. If you do not have a Half-Fare travelcard, you purchase a route class upgrade at full price. They are valid on the selected day until 5 a.m. the following day on GA-area routes.
More Info about Saver Day Pass you will find here: Saver Day Pass – book earlier, travel cheaper | SBB
Have a nice day, StéphanieB