Hello Eunice
It depends on the ticket. Is it a regular point-to-point ticket? If so, you can ask the ticket conductor on board to have it altered for you. This is possible on IR and IC trains. Alternatively, you can go to the train station in Davos (or any other staffed travel center) and ask for a change-of-route. This can also be done days in advance.
No, it is not possible to change it online.
Best regards,
Roland