Can I upgrade my iPhone's iOS from 4.1 to 4.3? I am currently using an iPhone 3GS. My phone's iOS version is 4.1. I want to upgrade to 4.3 but the latest version is 6.0. How can I upgrade to 4.3 or 4.5 from 4.1?
Simple answer is no it will not work.

Apple stopped signing old versions long ago; you either have to stay on the iOS you have or update to the latest your device will support.

A 3GS will support iOS 6.1.6 available via iTunes or direct from http://ipswdownloader.com