If it's "just" to update & test an airport:
- download and install WED from XPlane site and follow the advices ("fake" XPlane directories creation) on FG Wiki (search for "WED") ;
- make sure you download the latest .dat available for this airport on XPlane scenery site (
http://gateway.x-plane.com/airports/page ). Note that there is currently approximately a 2 years delay between the data on XPlane site and the data you can see in the latest FG scenery if you are using Terrasync. The latest data shipped in the next World scenery can be seen on the mapserver :
http://mapserver.flightgear.org/- update the airport via WED ;
- test it via
http://scenery.flightgear.org/tgweb/ (note that the elevation will probably not be good, as the airport tile won't be clipped with adjacent tiles) ;
- submit it to XPlane scenery gateway (you'll need an account first) and wait for it's update in FG.