Many years ago I would have muscled the AC onto the roof with sheer brute force and beer.
Now that I am older I do not drink beer and have a bad back from muscling things around with sheer brute force in my youth, I make a simple sled from plywood and a couple pieces of wood for guides and pull the AC up an extension ladder.
I do not trust my roof so use pieces of plywood to distribute my weight and try to stay close to the edges or over inside walls. Short pieces of pipe can be used to roll the unit around. PVC pipe works well, is easy to cut and cheaper that a chiropractor visit.