it's pretty easy, have the firmware you want to downgrade e.g 4.2 to 4.1 you must have the 4.1 OS 1st, then enter recovery mode by holding power+home button around 10seconds and then release the power button. next go to itunes and itunes will notice your iphone is in recovery mode, then click restore while holding SHIFT button if you're a windows user and browse the OS, mine's on the desktop , easy to find. it takes around 10 - 15 minutes to downgrade, and the last thing there will be an error number i forgot, but ignore that, the os is installed, to get out of recovery, use tinyumbrella, but you will notice the baseband cannot be downgraded