Just google the stand alone IPSW file. they highest you would be able to go is 4.2.1 Find the file, download it to your computer, then go into iTunes and right click on check for downloads on the summary page and it will let you specify which .ipsw file you want to use to downgrade the SW. If the device was jailbroken there is a chance the device may be bricked for good now just so you know. Good luck.
Answers & Comments
Just google the stand alone IPSW file. they highest you would be able to go is 4.2.1 Find the file, download it to your computer, then go into iTunes and right click on check for downloads on the summary page and it will let you specify which .ipsw file you want to use to downgrade the SW. If the device was jailbroken there is a chance the device may be bricked for good now just so you know. Good luck.