Unplug the microwave.
Discard the burnt bread in an outside trash receptacle.
Wipe down the inside of the microwave with a warm soapy water and sponge or cloth. Avoid using too much water by wringing the cloth or sponge out well. A good cleaning will help to get rid of crumbs and other debris that may perpetuate the odor.
Wipe away soap residue with a second damp cloth and leave the door open for 15 to 20 minutes to allow it to air dry inside before plugging it back in.
Cut half of a lemon into small pieces and add it to a microwave safe cup filled halfway full of water.
Heat the lemons and water for four to five minutes on high so that it is boiling.
Leave the cup in the microwave for an additional five minutes.
Wipe down the inside of the microwave with a clean dry towel and leave the door open for 5 to 10 minutes to allow it to air out again. Reheat the lemons and water again if needed to finish eliminating the smell.