You go to an OW airline ticket office or GSA of course, although most of us don't try to reissue until we have actually started the journey. You can do a search of the forum and find that people have reissued their tickets at LHR, LAX, NYC, HKG (a favorite place as CX was so efficient - now you have to pay an additional fee if the original tix is not on CX stock), NRT, etc. As for trying to reissue with another carrier prior to starting your journey I have doubts. Most likely you will have to start over by refunding and issuing a new ticket on the other carrier.