To replace an over-the-range microwave filter, pull or slide the filter out from underneath the oven, and clean it or replace it. Some microwave filters can be cleaned and others are even dishwasher safe, so check the owner's manual for specific instructions on replacing and maintaining the filter.
If the filter needs to be replaced, slide or pop a new one into the place of the old one. For filters than can be cleaned, fill a bucket or sink with boiling water, and add 1/4 cup of baking soda and a squirt of a de-greasing dish soap, such as Dawn. To mix the soap, agitate the water with a brush or long utensil. Then submerge the filter in the hot water, and let it soak for 10 minutes.
With a nylon or non-abrasive scrub brush, scrub the filter, and add more dish soap as needed. Once the filter is clean, rinse it in hot water, and dry the filter with a towel. Slide the filter back into position on the underside of the microwave.
Even if the filter is dishwasher safe, wash it manually whenever it is especially clogged and dirty. To keep the filter clean, wash it on monthly basis. A clean filter is important, because it prevents a sticky film from forming on the wall and appliances that are near the stove.