How do i upgrade from 3.1.2 to 4.0

[trentsmith326]

i already downloaded the 4.0 ipsw from felixbruns.com but apple changed something to where it has to verify the software your updating with before you can do it. I don't want to update to 4.1 bcuz there isn't a jailbreak for it yet. my ipod is a 2nd gen 8gb. HELP D: please

 

[Aero26]

iTunes has barred up/downgrade to any jailbreakme capable firmware.