Hello Rooghueremb13
If you need to change the entire last name (e.g. it reads Miller instead of Johnson) you must purchase a new ticket.
The name can contain up to three typos (e.g. Muller instead of Miller) provided the name is still recognisable according to your passport and a confusion with other passengers of your is not possible.
Supersaver tickets are not cancellable and cannot be refunded; regular point-to-point tickets, however, can be cancelled and refunded. In the latter case you csn simply cancel the ticket and repurchase.
https://sbb-tickets.cc/en/help-and-contact/refunds-compensation/refunds-compensation/ticket-refunds.html#anchor4
Kind regards,
Roland