Spilt or splattered food in the microwave can lead to damaged paint. As the food hardens, it carbonises and becomes difficult to remove, often taking paint with it. You can find paint made specifically for microwave oven cavities, but any gloss enamel paint will work and costs less. Choose a colour to match your microwave's interior. It is not necessary to paint the entire cavity at once, but you may choose to do so for a more uniform appearance.
Scrape off chipped paint and carbon deposits with a blunt knife.
Sand the area with a fine sandpaper to smooth the surface. You do not need to sand the entire interior of the microwave unless you plan to paint the whole thing.
Clean up the removed paint and dust from the interior with a microfiber cloth.
Apply paint with a small brush, or use a spray paint. If you can't paint immediately after scraping and sanding, you can still use your microwave, but the steel inside eventually will start to rust. You should paint as soon as possible to prevent rust.
Clean spills from inside the microwave to prevent paint damage.