Sure, no problem. Just take as much weight off the rear wheel as possible, then get a screwdriver or something that will just barely stick into the small hole in the adjustment cam just below the spring. Turn the cam (around the axis of the shock) and you'll feel it click into one of the 5 positions. You should also see the location of bottom of the spring changing as the preload changes.
If you run into any problems, let me know.. I happen to have my shocks off the bike (and for sale, in case you muck up yours
) and can easily take photos of what I mean.