See my reply in the thread 'Pitch Problems with Some Models..'
You arm yourself with Notpad or Wordpad so you can edit the aircraft.cfg file and AirEd.exe so you can edit the air file. It also helps to have a basic understanding of aerodynamics (like power, lift, drag and stuff)
Then you shut yourself away for a couple of weeks and you change things in the files - one or two at a time, then restart FS and see what effects the changes make. You either give up (many do) or at the end of it you learn about how to edit an fde - and you end up with what you're looking for.
LOTS of people ask how you do it. There are one or 2 tutorials around - but there's no quick fix. You learn by doing it because different factors affect different aircraft in different ways.
So that's it. It's not easy, it's also time consuming to learn and because you need to keep stopping and restarting the sim to see the results of your work (much of which you junk) not many people have enough patience to succeed and/or are good at it.
So basically you're on your own if you can't find a replacement suitable fde already done for you!
Best of luck and hope you come through!

Roger