How do I update my iphone as I have no update in the general setting

If you have an iPhone 3G as shown, it can not be upgraded beyond iOS 4.2.1.

Update it from iTunes on a computer as described at the bottom of this article. As it's running an iOS version prior to 5.0, there won't be an option to update from the device itself.

An iPhone 3G can't be updated past 4.2.1.

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If you have an iPhone 3G, as your profile states, iOS 4.2.1 is the end of the line for your phone. There are no further updates available.