Hello Feexieghee34
Good day and welcome.
There is no confusion, it is all said on the ticket:
Point-to-point, single journey.
Thus, it is an one-way ticket, Interlaken to Montreux. Period.
You can cancel this ticket and repurchase the day pass. Hopefully you purchased the ticket as a registered user so the cancellation will be free of charge.
Kind regards,
Roland