Okay, the *.mdl file is the model of the aircraft. The is essentially the shape that shows up in the sim as the aircraft. You cannot edit this without having the source files the model was created with. These can only be gotten from the author.
The *.air file contains all the information about an aircraft, such as the wingspan, weight, size, engine and many other variables, most of which are overridden by the aircraft.cfg file. You can edit this with several editors, available for free. The most popular is AirEd, found here:
http://perso.wanadoo.fr/hsors/FS_Soft/fsairfile.htmlThe aircraft.cfg file is easier to edit than the *.air file, but it doesn't contain all the information the airfile does, such as the wing shape and others. But it can be edited easier, and all you need is notepad. Just open it and take a look. Most of the sections are easy to understand, while others take some time and trial and error to get correct.
A WORD OF WARNING!
Don't change anything in the *.air files or the aircraft.cfg files unless you back them up first! Just make a copy of them in the same folder by right-clicking on them and then clicking COPY. Then click anywhere in the same window and right-click/PASTE. This will ensure that anything you do to the files you can undo simply by replacing them with the copies.
If you have any more questions, feel free to ask here, or IM me.