I really want to upgrade this airport, because it's causing some big backlogs for me. However, it's so busy that there are constantly airplanes either loading/unloading, in the hangar waiting or circling to land. This means I can't delete to rebuild. Is there any way I can clear it out temporarily to allow me to demolish/rebuild?
You can try to have all the planes dock somewhere else, which would take a lot of time.
A possibly easier thing to do is to build a temporary airport to accomodate a place for the aircraft to be during the upgrade. What I mean with that is that you replace this airport with the temporary airport in the airplanes directions.