How to Update iPhone 3G to iOS 4.2.1 without Losing Jailbreak & Unlock?

Hello Gilbert, updating, jailbreaking, and unlocking your iPhone 3G on iOS 4.2.1 is possible. But first, since your iPhone 3G has iPad 6.15.00 baseband, you should be noted that you will need to use either Sn0wbreeze (for Windows users) or PwnageTool (for Mac OS X users) in order to create a custom firmware for your iPhone 3G. To do that, if you are a Windows users then you can download Sn0wbreeze 2.2 using this download link. On the other hand, if you are using a Mac OS X then you can download PwnageTool 4.2 from here. You will also need to download the stock iOS 4.2.1 IPSW file for your iPhone 3G which can be downloaded from this blog post.

Once you have downloaded the required tools and software to jailbreak iOS 4.2.1, you can now start creating custom firmware for your iOS device. You can follow this tutorial on how to use Sn0wbreeze 2.2. You can also refer on this guide on how to use PwnageTool 4.2 on Mac OS X. Lastly, you can use Ultrasn0w 1.2.3 to re-unlock your iPhone 3G. The step by step guide on how to unlock iPhone 3G can be found here.