Dawn dishwashing soap breaks down and removes grease.

I put a glass measuring cup filled with water and about two tablespoons of lemon juice and heat it on high for five minutes. Let it sit for about 10 minutes and then remove it (I pour mine down the kitchen drain) and wipe the microwave with a microfiber cloth and leave the door open until it's dry. Do this whenever you see any buildup, or once a week and you won't have a problem with grease.

Add to Cynthia, I also spray it a bit with white vinegar after the Five mins. Use Dawn too.

https://www.hometalk.com/29728727/easiest-way-to-clean-a-microwave

Boil water in the microwave, let sit 5 minutes, then wipe clean. If greasy on outside, 'like dissolves like'. Wipe with coconut oil, let rest or soak, then wipe clean with warm, soapy cloth.

The best way to clean your microwave is put a cup of water with lemon juice in it until is boils. This will soften any hardened splashes. If you don't have lemon juice, use vinegar instead. After that just wipe it down with any dishwashing soap and water.