Download iOS 4.2.1 IPSW
On November 22nd, 2010 iOS 4.2.1 (marketed as 4.2) was publicly released to the iPhone (3G, 3GS, 4), iPad, and iPod Touch (2nd Gen or later). As with previous iOS releases, not all devices are compatible of the same features. iOS 4.2, designed for iPad, enabled the iPad to use the key features of iOS 4 (Multitasking, create folders, use Game Center).
By updating iPhone OS to iOS 4.2.1, you will update the baseband version to 05.15.04 (3G and 3GS) / 03.10.01 (iPhone 4) basebands.
Download iOS4 (4.2.1) IPSW file for Apple iPhone 4/4G, iPhone 3GS, iPod Touch 2G, iPod Touch 3G and iPod Touch 4G, also iPad directly from Apple’s server.
If you don’t know how-to install iPhone firmware to your iPhone, read this guide: How-to Update iPhone firmware Shift + Restore method