Is this an award ticket or a revenue ticket ? If award, indeed you cannot modify : you need to cancel and rebook which will likely mean that you won't be able to rebook if sold out (and possibly overbooked). If this is a revenue ticket, I can't see why your ticket cannot be changed without canceling : the 1st leg can be removed and the whole ticket repriced.
If you booked through an OTA, you need to deal with them.