Ken,
This is fairly easy. Remove the trim ring around the window (screws are right on the face). The screen is held in place with a old fashioned cord type screen retainer. Just pull that small cord out of the groove around the screen frame holding the screen in place. Replace the screen material and tuck the cord back in the groove to hold the screen back in. This tucking in can be done with a flat blade screw driver but is easier if you have a window screen roller. Window screen rollers are that little tool that looks like a small pizza cutter, you can get these at your local hardware store. Reinstall the trim ring and you are done.
Cary