How do I change an external door to a window?
You will need to remove any rotting wood, find any water intrusion areas then you can use 2X4's or whatever you need to fill in above the floor up to the bottom of the window. Check on the rough opening size needed for the new window and frame accordingly. The interior is easily covered with drywall or whatever. The exterior is a little more tricky. You might want to match the existing siding material if that's what you have.