Just remove the drawer below the microwave and remove the 4 bolts holding it in place.The old unit just slids out.You need to remove the rubber feet on the new unit, insert it into the opening(no mods req'd) and the re-install the rubber feet from below.A very easy install.
we just replaced our TAPPAN with a Breville 3-in-one combo.Just had to remove the rubber feet to get ot to slide in and then reinstalled them from the bottom once installed.