Fireplug, you can either exit the plane, or go to the rearm point to keep the same mission going.
To exit, first make sure the plane is completely stopped. Then type / on the keyboard to open the radio and type
.ef
That's a period at the front.
You can also bring up the clipboard by pressing the Esc key and click on End Mission.
To refuel/rearm and continue the mission, you have to taxi to the rearm point. The rearm points are at the end of each runway, near a small building. The rearm point is inside the big yellow circle on the ground. You may want to recon the rearm points with a jeep first, just so you have a better idea of where they are.
Just a warning: you need to be pretty good with the rudder to rearm. I've ripped a few too many wings off trying to exit the rearm point.