Gotta take the big nut off, and then the sprocket will be loose, you'll have to loosen the chain first or you'll never get it off.
The big nut is pretty much impossible to get off if you don't have the right tool, either a big ass socket and impact wrench or breaker bar, or a big ass wrench. I usually use my 18" adjustable wrench, just put it in gear or all you'll do is make the quad roll.
You shouldn't replace the front sprocket by itself. The sprockets and chain wear together as a set, and if the front sprocket is worn badly then everything else is going to be pretty worn down too. If you mix new and old it'll accelerate wear on both the new and old stuff because the patterns won't match, and you run a much greater risk of having the chain jump off and destroy the engine case. It happens a lot and it's not worth risking destroying the engine case to save $50.
