How to download ios 4.3 on an iphone 3g?

I updated my IPhone 3g and after I updated it, it deleted all my apps and contacts. So i went on the app store, and i tried to install Facebook and it said i needed to have IOS 4.3. But i don't know how to update it.

Man this is a common question today. Nope.iOS 4.2.1 is the last on the iPhone 3G. You'd need to get an iPhone 3GS at least to get iOS 4.3 (goes up to iOS 6.0.1 which is the latest)