Whether your microwave just stopped working, or you upgrade to a newer model, you must properly dispose of your old microwave. Some states, such as California, regard certain microwaves as hazardous waste. Other states do not consider microwave ovens as hazardous waste, but they do require the microwaves be recycled. You can't, therefore, just throw the old microwaves out with the garbage on trash day.
Offer your old microwave to someone who is in need if the microwave is still in working condition. Place an ad in your local paper, or post the offer on your town's Freecycle page (see Resources). Any takers will contact you and arrange to pick the microwave up. Alternatively, donate your working microwave to a local charity.
Call your local waste management facility. They will know the town's laws, and can look up what the requirements are for disposing of microwaves that no longer work properly. You may need to provide the waste management official with your microwave's model number, so have it handy.
Take your old microwave to the recycling centre or hazardous waste collector that your waste management official recommended when you contacted him. Some centres accept these small appliances for free, and others charge a fee. Your waste management official will inform you of any fees, or give you a direct phone number for the recycling centre or hazardous waste collector so that you can ask them about their current fees.
Consider selling your old microwave to someone who fixes old microwaves and resells them for a profit. This keeps the microwave from being tossed into a landfill.