It sounds like what you want to do is cancel your flight to your destination and only fly the home bound portion.
That isn't going to work. Changing and canceling are different. If you cancel the first flight your round trip airfare goes away and you will then need to buy a one way fare home. Now that may not cost more then your original round trip or it could cost LOTS more. Just depends on the airlines.
Also, do not just not show up for your first flight and then try to take the flight home. It won't be there. When you don't show for the first flight the reservation is cancelled.