I am using iphone 3GS. My phone os 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?
Download the firmware iOS 4.3.5 from Apple:
and then perform the update with this file. You select to install from a local file by holding the Option key while pressing "restore iPhone" button in iTunes.
You can find a list of all available firmware files at various locations – use your favorite search engine to search for "iphone firmware files" and you'll find e.g. these lists:
Jump over to this website http://www.felixbruns.de/iPod/firmware/ which has every iOS for every model of phone. Simply find the correct model for your phone and click on the iOS version you want and download.
Once downloaded, follow the instructions on the top of the site (copypasta below)