No, you can’t. Right off the bat, in order to replace your AC unit you’ll need to be able to handle refrigerant. For that you’ll need an EPA license and you can’t get that from a book. Though I suppose it may give you the know how to pass a test. The problem is, if all you want to do is replace your AC/furnace, you’re not going to save money this way.
Replacing a furnace is somewhat straightforward. There’s a bit of an art to disconnecting and reconnecting the plenum, but other then that, it’s not terribly difficult. You can probably do it without even breaking the lineset. But, if you’re going to do any refrigerant work at all (replacing your outside or inside unit), you’ll probably need, let’s say, one to two thousand dollars worth of tools just to get the freon out, store it and put it back in when you’re done*. If all you want to do is get the job done one time, you’re better off just paying someone to do it for you. Even if you get ripped off it’s better then buying all the tools and then having them sit around in your garage for years.
*Look for prices on a vacuum pump, recovery pump, storage tank and HVAC gauges.
ETA you can buy and replace your furnace on your own, no special license required.