Hello linda_1994
In general, we can cancel your ticket and you can just buy a new one for another date. The conditions of reimbursement depend on the ticket you purchased.
If you have buy the ticket in a SBB station, you have to return at the counter to do the change.
If you have buy it online, you can contact our ticketshop: ticketshop@sbb.ch / 0848 030 030.
Kind Regards,
CharlineB