Hello,
International train tickets can be changed or refunded (as long as ticket conditions permit it) at the phone or at any SBB counter. You can call us anytime at 0848 44 66 88 (CHF 0.08/Min) or, if you call from abroad, at +41 51 225 78 56. And here you will find the opening hours of our stations.
I wish you a lovely day and safe travels,
Chiara_S