[How-TO] - Download iOS 4.2.1 ipsw

You could update your iPhone 3G/3GS/4 or iPod Touch 2G/3G to iOS 4.2.1 using iTunes. You could download iOS 4.2.1 IPSW files from the below given direct download links by Apple.

iPhone 3G
iPhone 3GS

iPhone 4

iPod Touch 2G

iPod Touch 3G

iPod Touch 4G

iPad