How to update my iOS on my iPhone?

I have an iPhone 3G and it is running on 4.2.1 iOS. When I plug it up to iTunes it says that it is updated, but clearly there are newer updated versions of the iOS. Does anyone know how I can update my iOS?

Do it from the iphone.go settings-general-software update

You can't 4.2.1 is the highest firmware for the iPhone 3G

Apple no longer supports the following iOS devices:

OG iPhone & iPod touch (1st gen): iOS 3.1.3 is the final version
iPhone 3G & iPod touch (2nd gen): iOS 4.2.1 is the final version
iPod touch (3rd gen) and OG iPad: iOS 5.1.x is the final version

I have listed links of devices supported under both iOS 5 and the upcoming iOS 6. You need an iPhone 3GS or later for both iOS 5 and 6.

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