It's pretty messy. You need to go to a site that has the ipsw file that you need and when you plug ur phone into the computer hit restore while holding alt and chose the downloaded ipsw. This will erase everything on your device and coud ruin it so i highly am against it. If you are looking for a jailbreak just use greenpois0n wich works with 4.1.2 untethered and fast.