Hello Bietale66
Please kindly note, international tickets cannot be changed but cancelled and reissued. As far as I know,if you want to alter the ticket partially, this only works if you cancel the entire trip.
Provided you bought it with SBB CFF FFS, please get in touch with the SBB Contact Center by phone (+41 51 225 78 56 / 24/7, waiting times may occur), or pass by an SBB Travel Center.
More information is available here: Help with international tickets for Europe. | SBB
Kind regards,
Roland