Hello Vielienuz80
You have to purchase a new Saver Day Pass at full fare first, then you might be eligible to ask for a refund of the half fare pass. The newly purchased pass must be for the same day and the same person.
You can find the refund form here. Please state in your refund request the e-ticket number of your new Saver Day Pass:
https://www.sbb.ch/en/help-and-contact/refunds-compensation/refunds-compensation/refund-form.html
The final decision on whether the refund request is accepted lies with SBB.
Kind regards,
Roland