Hello elysaa
If it concerns the Swiss Travel Pass or a standard point-to-point ticket, you can cancel it anytime and repurchase. A CHF 10 administration fee is applied, however:
https://www.sbb.ch/en/help-and-contact/refunds-compensation/refunds-compensation/refund-form.html?stepId=3.17
For a Saver Day Pass or a Supersaver ticket, you have to see with SBB if they will cancel it, if at least all other information is correct and only the year is wrong. Simply attach the ticket and also a copy of the person’s passport:
https://www.sbb.ch/en/help-and-contact/customer-service/praise-comments/claims-complaints.html?stepId=3.17
Usually, such tickets are not refundable. There is no guarantee that SBB will cancel your ticket, whatsoever. The decision is with them.
Let us know about the outcome.
Best wishes,
Roland