Using a #1 Phillips head screwdriver, turn the screw in the troopers leg counter-clockwise to loosen and remove. The screw is pretty small, make sure you don't lose it!
More often than not, once the screw is removed, the
outside part of the upper thigh will give way freely. Occasionaly you may have to pry it with a small flat head screwdriver. Always pry the outside thigh away from the inside thigh should you come across this.
To remove the inside thigh: Turn the inside thigh so you can see the ball joint that the thigh sits into. simply hold your thumb against the thigh right beside the ball joint. Place your index finger at the knee part of the thigh and pull up with your index finger. The thigh will pop right out of the socket.
Make sure you keep track of which parts go to which leg. You could do them one at a time if that helps you keep orgainized. Reverse the process to attach new leg from one of the silver sigiled troopers.
Once your done your trooper should look like this.