Help changing destination on multi-city ticket

[danielc]

I have a UA economy nonrefundable ticket as follows: MEL-LAX-MHT, then MHT-SFO, and finally SFO-LAX-MEL commencing on 28/10/14. My plans have changed and I now need to get to Spokane (GEG) instead of Manchester. All travel dates, etc remain the same. Looked to change it online but thought I might get some advice before messing with the booking. Any suggestions on the best way to change at the least cost greatly appreciated.