That's the problem, I cannot change the AFCAD. There is no option to select.
You have an AFCAD conflict, caused by the Traffic product you use. It's useless being able to "change" the AFCAD in GSX, since even if you could do that, you would still have the conflict *in the sim* so GSX is just telling you which file is in use, so it will be your decision what to do with it.
If you have an add-on scenery for an airport, those traffic AFCAD files should be removed for it, since they are only supposed to be used with the default scenery, and they almost invariably cause conflicts with add-ons, so the best solution is to simply remove them.