You should clean the microwave very well. Then you should spray liquid cleanser should on all surfaces and then wash it off.
Preferably you should remove and replace the glass turntable with a new Kosher for Passover turntable. Sfardic Jews who rely on the opinion that glass does not absorb can wash it well (or immerse in boiling water after next step) and needn’t replace the turntable.
You should leave the microwave for 24 hours and then place a clean drinking glass , unused for 24 hours, filled with water and boil it in the microwave for 10 minutes (Note: It take several minutes for the water to begin to boil so you need to add those and have ten minutes of the water boiling.)
Repeat the boiling process in another part of the microwave. Ashkenazim should place a piece of cardboard or contact paper over the glass window pane. Sfardim who hold that glass doesn't absorb needn't cover the glass.
According to the opinion that one doesn’t kosher plastic for Passover, then you should cover all surfaces with cardboard or contact paper leaving holes for the vents. But you can consult with your rabbi as to if you can rely on the lenient opinion and not cover it.