There should be two bolts going down through the bottom of the cabinet above the unit. After removing them, you should be able to tip the unit downwards and the back of the unit will unhook from the wall bracket. Not sure about finding another one with the same specs, but hopefully you'll be able to get some measurements once you have it down.
Best to have two people when do this there pretty heavy, once those two bolts are out, drop the front and lift up up the back and it should slide right on out.
Microwaves come in pretty standard widths so it should not be a big deal to find one. As long as the width is right it will work. Your going to have to read the directions to make sure the baffles in correctly to make it blow air out the back or the top of the unit.
There's going to be a template in the box to use to see if the old and new duct work will line up.
The new one with come with all new mounting brackets so you can toss the old ones there not going to fit the new one.