Unplug the microwave from the wall outlet.
Open the microwave door.
Pry the plastic ring on the inside of the door off using a flat-head screwdriver or butter knife. Be very careful to not break the plastic ring while prying.
Remove the two screws on the inside of the door securing the handle to the door, then remove the handle.
Align the new handle with the mounting holes, then secure the handle using the screws that were removed from the old handle.
Snap the plastic inside ring back into position, then close the door and plug the microwave back into the wall.