By Horav Yosef Yeshaya Braun, member of the Badatz of Crown Heights
Kashering a microwave from milchigs to fleishigs or from fleishigs to milchigs on a regular basis is not appropriate, since it is a generally accepted custom not to kasher regularly back and forth.
However, if inadvertently fleishigs was put into a milchig microwave or vice versa, the halachah is that one should wait twenty-four hours until the microwave is no longer a ben yomo, and then it can be kashered.
There are two ways a microwave can be kashered, and preferably one should employ both methods.
One method is to first cover/seal the vent and then pouring boiling water on all of the microwave’s surfaces.
The second way is to place a cup of water into the microwave, put it on the highest setting, and let it heat up for a minute or two until the inside of the microwave is full of steam.
Since the area under the cup of water did not get kashered, this process should be repeated with the cup placed in a different spot.
Many are stringent and maintain that the glass spinning plate should not be kashered.