That's pretty basic but it's hard to describe over a computer. You need someone standing next to you who can point and describe. Normally I'd say something like "unbolt the old one, pull it off, put the new one on and bolt it in place", but there's more to it than that. You have to disconnect one battery cable first to prevent sparks. Do you know why we always recommend removing the black negative cable? Do you know how much to tighten the nuts on the little wires so you don't break the studs? Do you know which way to turn nuts and bolts to loosen them? I don't mean to sound sarcastic but if you have to ask how to do something simple like replacing the generator, that implies you haven't learned the basics yet and we are going to incorrectly assume you know all of the potential hazards involved.
If my assessment is right and this is new to you, having the manufacturer's service manual is your best bet. It will list every little detail and provide warnings of what to watch out for. Next, you should have the new generator at hand to look at for reference. When you don't understand something, post a follow-up reply here with the specific questions.
If your question is related to a bolt you can't find or there's no room to get the generator out, a '68 model is a little before my time, but that was rarely a problem back then. I'm visualizing a generator near the top of the engine and easy to reach. For me the hardest part is removing the power steering belt if it has one and it has to come off to get to the generator belt. I always have trouble finding all the nuts and bolts to loosen the power steering pump.