How can I upgrade the plane at McKenzie Airfield?
Solution 1:
I'm afraid that the plane you use for the package drop/bombing missions from the Mexican guy as Trevor is what gives you the mission. You can not use a different plane for this, in the same way that you can't use anything other than the off-road buggy for the ground-based arms race missions.
Solution 2:
No sorry you have to put up with that plane. The best you can do is save a different airplane there but you can't use that for missions