Connect to iTunes on the computer you usually Sync with and “ Check for Updates “...
If an Update Appears Install it... if not... you are up to date for your particular Device...
See the Using iTunes Section Here...
The iPhone 3G can Only Update as far as iOS 4.2.1
Identify iPhone Models
Update it from iTunes on a computer as described at the bottom of this article; devices running an iOS version prior to 5.0 can't be updated straight from the device. It will be updated to the latest compatible version.
The iPhone 3G and second generation iPod touch can’t be updated past 4.2.1.
Are you sure you have an iPhone 3GS and not a 3G?
The latest version for the iPhone 3G is iOS 4.2.1.
If you have a 3GS, you can only download the latest version, currently iOS 6.1.3; you cannot download an older version like 4.3. To update, connect to iTunes on your computer and when it detects your phone it should let you know that there is an update available.