Hi porthos82
Thank you for your request.
It is not possible to modify the tickets. You have to cancel them and buy new tickets. Attention, cancellation fees are to be paid in this case.
If you bought them at the station, you have to go to the ticket office to cancel them. If you bought the tickets online (SBB Ticketshop), please contact directly our Railservice (+41 848 44 66 88).
My colleagues will be able to help you.
Best regards
NicolasK