i need to update to ios 4.3

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.

The original iPhone and first generation iPod touch can’t be updated past 3.1.3.

The iPhone 3G and second generation iPod touch can’t be updated past 4.2.1.

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