use redsn0w to perform the upgrade. Works good for me. Otherwise you have to use itunes, put phone into DFU mode, then itunes will tell you to restore, but hold shift button down and click restore and then browse to where you downloaded the 3.12 software and choose it and itunes will install it. NEver just click restore and take the software itunes wants to d/l and give you. This way you are manually choosing what to install.