Hello KJM
Welcome to the SBB Community.
It is not possible to alter tickets. If you need to have changed the validity dates of your ticket, the current ticket must be cancelled and repurchased.
Provided your ticket was purchased from SBB, to cancel the ticket, see:
https://www.sbb.ch/en/help-and-contact/refunds-compensation/refunds-compensation/ticket-refunds.html
A CHF 10 fee is applied to cancel your Swiss Travel Pass.
Kind regards,
Roland