Unplug and open the door, mix up some baking soda with a little water and spread it over the window. Let it sit for 20-30 minutes. If it seems to be drying out, spray with water or put a wet paper towel over the window. After the 20-30 minutes, scrub with a cleaning cloth or a plastic scrubbie if you have stubborn stains. Then rinse off the baking soda using the cleaning cloth, repeatedly rinsing it in water.