If you use an Advance Pass for commuting to work and need to change one of your bookings, you can do this directly from the Passenger Portal.

Open the Passenger Portal and hit the "Details" button on your Advance Pass. On the ticket details view, hit the "Bookings" button.

The bookings view will list any trips you have already booked. Bookings that are for the next day and beyond include an "Edit" button.

Hit the "Edit" button on the trip that you wish to change. On this view, you will be given the option of the service you instead want to select and the day of travel. Tick the appropriate option, and then hit confirm.