Hello Peamairrai55
I suppose that you have a ticket from Milan to Switzerland. If it is exchangeable or refundable, you can call the SBB Contact Center with this number:
+41 51 225 78 56 (24/7, waiting times may occur).
If you have an Arcobaleno Transfrontaliera Ticket, it should also be possible to cancel it in sbb.ch online and buy a new one.
Kind Regards
Danhiel