While it's a relatively low-maintenance appliance, your microwave does require some occasional attention. As long as you clean up any spills or messes as soon as you make them (or see them), and wipe down the interior and exterior every so often, you don't have to spend a lot of time keeping it in working order. That said, you should be cleaning your microwave's grease filter on a semi-regular basis—depending on how much you use it. Plus, your microwave might also have a charcoal filter, which can't be cleaned. Here's how to locate both types of microwave filter, and either clean or replace it.
How to locate and clean a microwave's grease filter
First, only over-the-range microwaves have filters, so if you have a countertop model, you're off the hook.
A microwave's grease filter is typically located on the underside of the appliance, and simple to remove: Just be sure to unplug your microwave before getting started. Most grease filters will slide right out, though some models may require you to unscrew or unclip it first. When in doubt, check the owner's manual.
Once the filter is out, fill your sink or a basin with hot water, add a squirt or two of liquid dish soap, and let it soak until you can easily wipe off the gunk. If the grime isn't budging, add 1/4 cup of baking soda to the soapy water, and let it soak a little more. You can also try gently scrubbing it with a dish brush. When it's finally grime-free, rinse the filter with clean, warm water, then let it air dry before replacing it and plugging the microwave back in.
Most manufacturers recommend cleaning your microwave's grease filter every one to three months, though again, it depends on how much you use the appliance.
How to locate and replace a microwave's charcoal filter
Most over-the-range microwaves use a charcoal filter in addition to the grease filter—or at least give you the option of using one. Ideally, microwaves are vented so that the exhaust goes outside, but depending on the configuration of your kitchen and home, that may not be possible. If that's the case, and your microwave is vented inside, a charcoal filter is useful for removing cooking odors, and should be replaced roughly every six months.
To do this, start by unplugging the microwave. Most charcoal filters are located on the top of the microwave, behind a vent panel. If you don't see yours there, consult your owner's manual.
Most vent panels are held in place by screws, so you'll have to remove those before lifting the panel out, then removing the charcoal filter. Dispose of the old filter, and replace it with a new filter compatible with your microwave model. This is also information you'll find in the manual. Once the new charcoal filter is in place, screw the vent panel back on, plug your microwave in, and get back to reheating leftovers.