Buena
Of course there is a ticket office in Switzerland’s largest railway station:
https://sbb-tickets.cc/en/travel-information/stations/find-station/zuerich-hb-station/shops/shop-detail.html/geo-billette-abonnemente-freizeitangebote-695b
Dates can generally not be altered after a ticket has been issued, only the entire ticket can be cancelled and get refunded under some conditions. But as the validity of that return ticket has already started by now, a refund would not be possible anymore according to the rules. (You could already be using that ticket right now if having stayed the night on Jungfraujoch).
Nevertheless go to the counter, remain friendly, and hope for some goodwill from the staff, without expecting it.