Hello Puneite
Hi and welcome to the SBB Community.
If you hold a 2nd class Swiss Travel Pass and you want to ride in 1st class, you can purchase a class upgrade ticket (either for a route or a day pass) at ½ fare. How to set the half fare option in the SBB Mobile app we described here:
https://community.sbb.ch/d/26418-how-can-i-save-the-info-of-swiss-half-fare-card/2
All clear?
Here is an overview on all possible class upgrades:
https://www.sbb.ch/en/tickets-offers/tickets/point-to-point-tickets-switzerland/class-upgrades-supersaver-class-upgrades/class-upgrade.html
Note: Travelling with the Swiss Travel Pass, aa with any e-ticket, requires your physical passport upon ticket check. Passport copies or photos on smart phones are not accepted.
Kind regards,
Roland