A few years ago I replaced an older roof top AC with a newer model. I did it by myself but it would have been GREAT to have had some help. Be sure to clean the old gasket off and replace it with a new gasket. After all that work you don't want water seeping in when it rains. It was pretty straight forward on mine - I removed the inside vent/grill and there was a connector box for the AC wires and the bolts securing the A/C. I disconnected those wires, and the inside bolts that hold the AC down. I was able to lift the old one out and put the new one in. My A/C did not have a wall thermostat so I did not have those wires to "mess" with. I placed a couple pieces of card board on the roof as a cushion for the old and new units. I could slide the cardboard and move the A/C's without damaging the roof material.
Bob, Sandi & Marmaduke the Big Pug
SW OREGON 2004 Journey 39K, 330 Cat
If towing: a 2006 Mini Cooper or 1995 Wrangler