Hello GG98
You mean a same-day return ticket where both trips are in one ticket? Or two single tickets? This is not clear from your question. Best to let us know the route you are going to travel…
If both trips are in one ticket, you have to cancel the complete ticket and repurchase for the single trip.
As a general rule, since we do not know the ticket type and route: a refund is only possible for regular fare tickets, not for Supersaver tickets. Also international tickets cannot be cancelled online but by phone.
https://sbb-tickets.cc/en/help-and-contact/refunds-compensation/refunds-compensation/refund-form.html
Kind regards,
Roland