If they are saver’s tickets bought online, you can return them within 30 minutes of buying them, and after that no more at all (except in case of proveable illness or death)
If they are regular online-bought tickets whose validity has not begun yet, you can return them on your own online.
The links for both of these should be in the confirmation e-mail which you have gotten after you bought them.
For paper tickets, you need to go to an office of SBB and you will most likely not get back the full price of the tickets.
In other cases please specify which ticket exactly you bought.