Hello RoanBengel
Whether international tickets are cancellable and eligible for a refund depends on the international tariff.
If they are cancellable at all, then it can only be done by calling the SBB Contact Center at +41 51 225 78 56 (24/7, waiting times may occur), or by personally seeking advice from an agent at an SBB travel office in a station. It is not possible to place a request online.
Kind regards;
Roland