i have a iphone 3 and i dont know how to update it

The iPhone 3 can be upgraded to iOS 4.2.1. Which version do you have?

Follow the instructions in the article you asked this question from; if it's running an iOS version prior to 5.0, it can only be updated from iTunes on a computer.

An iPhone 3G running 4.2.1 and an iPhone 3GS running 6.1.3 can't be further updated.

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how do u go about doing it from a computer I need help

Install the latest version of iTunes on your computer. Plug in your device to your computer. In iTunes, select your device. In the Summary pane, click Check for Update. Click Download and Update.

Update the iOS software on your iPhone, iPad, and iPod touch - Apple Support