Hello Quynh
Since you already purchased your ticket with the half fare reduction, you have two options now.
The first option is, you can purchase another half price ticket for the same trip. (only possible if the existing ticket is a normal price ticket - no supersaver ticket)
Your second option would be to purchase a new full price ticket and ask the refund of your wrong ticket. Please use this [form](http://Die%20Rail Checks können nach dem Ablaufdatum nicht mehr akzeptiert werden.) for your refund request. The refund will be done with a fee of 10 Franks.
I wish you an excellent time in Switzerland.
Regards RahelW