How do i replace the inside light in the microwave?

If you see no panel on the inside of the unit, you will have to remove the housing.
If it isn't high-mounted, this isn't difficult; there are 3-5 screws around the edge of the back panel that keep the metallic housing in place. Once these are removed, lift the rear flange of the housing and tilt it forward, then pull it back toward you to release the front lip.
As a precaution; unplug the oven overnight since there are components that can keep a nasty electrical charge for hours, maybe not life-threatening but highly unpleasant.