First mix some baking soda with some water, doesn't matter what proportions, and microwave on HIGH for about 3 minutes. Leave the container with the hot mixture in the microwave for about 30 minutes. Remove and wipe the inside down and wash the turntable, if there is one, in hot soapy water. That should help a lot.
If the baking soda doesn't work, try the same thing with a mixture of lemon juice and water, about 50/50. See if that helps.
If neither of these work, stuff the whole microwave full of crumpled newspapers, close the door and leave it closed up for about 24 hours. Be sure to lock the power ON switch so no one accidentally "nukes" the newspaper and starts a fire.
These are the only remedies I can offer.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!