Hello yong2
You probably purchased a Swiss Travel Pass, not a SwissPass.
There is no other way to cancel an international ticket (if cancellable at all) than by phone or physically in an SBB travel centre. The phone number is +41 51 225 56 78 (24/7, waiting times may occur).
If the ticket is a flex ticket that can therefore be cancelled, and you wish to cancel it once you are on-site, you will probably not be able to book another ticket for the same train as the train might be sold out. And, you will pay a cancellation fee. Whether you will indeed save money while going through all that hassle of the cancellation process (again, if feasible at all)? Frankly, I am unsure about the amount you will finally save in the end… I, personally, would simply use the ticket as it is.
Kind regards,
Roland