Well that depends what you mean by changing the stock planes. if you want to add more bomb or rocket selections, or make your guns stronger, you should download DpEdit here at simviation or get a DP tutorial to learn how to do it manually with notepad. Now, if you wanna change how fast or heavy a plane is, you should modify the .cfg and air file. if you want to change how a plane looks on the exterior, you cant. There is no way to reverse the MDL file and make it into a gmax source file or anything else.