How do I change the blade on a snapper lawn mower.

I do not know what model you have so lets start with assuming it is a single blade push mower. Remove the spark plug wire and rotate the blade to the discharge shute. Block the blade with a 2x4 in the shute and back off the bolt holding the blade with a breaker arm/socket or ratchet.

Since ballengerb1 told you on the push units, I will try and instruct you on the older style, (even though they still sell them new also resembling to older ones):
1. disconnect and remove the battery.
2. put the mower into park, or first gear and lock the brake.
3. grasp the front end of the mower and lift it up to stand it up vertically onto its back support bars.
4. use a wrench on the top side of the blade on the nut, and an impact wrench on the bolt head
5. remove both bolts( the two closest to the center bolt)
6. be careful, the blade will fall as soon as the last bolt is removed sometimes, not all the time.
7. remove blade and sharpen and balance, or replace as needed.
8. to reinstall, reverse directions, and allow unit to set back level for at least 15 minutes before restarting unit.