How to upgrade my iphone 3g to ios 4.3.1?

Specialy when most of appls and whatsapp are supported only on 4.3.1.
Kindly explain in detail along with steps and screenshots
Current version is 4.2.1

Added (1). People who think its straight update its not(please gimme a break). IPhone 3g only can be updated till ios 4.2.1. I would like to know any out of the box ways to do it (Jailbreak or watever)

U dumm

Just connect your phone to iTunes and choose to upgrade from your device.

It's fairly simple and straight forward. Many people on youTube have posted tutorials as well.

If you go to settings then general there would be a popup for a new ios upgrade if you can't then its too out of date since it is an old model