First: Go to Mojang.com and look for the latest snapshot and download the Client version.
Second(Windows xp): Click the windows button and R at the same time and type in %appdata% and click enter.
Second(Windows 7/Vista): Click the windows button and in the search bar, type in %appdata% and click enter.
Third: Look for a folder called .minecraft and open it.
Fourth: Look for a folder inside that folder thats named bin and open that.
Fifth: Make a folder on you desktop and name it "Minecraft Backup" and drag the minecraft.jar from the bin folder to Minecraft backup. (optional)
Sixth: Go to where you downloaded the snapshot and drag it in the bin folder to replace the old Minecraft.jar.
Seventh: Close all the windows and start minecraft up.
Enjoy...