When you connect your iOS devices to iTunes it automatically prompts for an update if you are connected to internet. In case if you don’t have access to internet and looking for ways to upgrade iOS to 4.3.3 then here are the links to download the installers directly from Apple Servers.
Links to download iOS 4.3 for iPhone 4
Links to download iOS 4.3 for iPad 2 & iPad
Links to download iOS 4.3 for iPod