Hello Leanoshill73
In order to upgrade you simply have to purchase another upgrade ticket, the same way you purchased your tickets:
Enter the route, select the upgrade option, select the passengers, done.
You can also purchase a Supersavrr Class Upgrade, if you can certainly meet the departure time.
It would probably have been cheaper to initially purchase a 1st class Saver Day Pass instead to upgrade it later. I personally would not even purchase an upgrade. Depending on the day & travel time, the trains have a low frequency from Bern to Visp.
Kind regards,
Roland