Time. Time will dissipate the burned smell.

There is airflow between the cooking cavity and enclosed mechanical/electrical area of microwave ovens. Odor/soot from a burned-food incident will get into that area which is inaccessible to cleaning without disassembly*. It should dissipate over time as Theresa Peterson advised above.

*Microwave oven outer shells typically are assembled with security fasteners due to the high-voltage capacitors/components and potential exposure to microwave radiation if run with the shell removed.

I have had luck with placing a half filled custard cup of ground coffee inside the microwave, closing the door, and letting it sit. Works well in the refrigerator, too.