After you’ve downloaded your device’s respective firmware, you can install the software manually by launching iTunes, locating your device on the sidebar and pressing “Restore” while holding the Shift key on Windows and Alt on the Mac.
Download iOS 4.3.4 for iPhone, iPod Touch and iPad
As expected, Apple has pushed out new i,e iOS 4.3.4 for iPhone 4, iPhone 3GS, iPod Touch 3G. iPod Touch 4G, iPad 1 and iPad 2 that fixes the PDF exploit discovered by Comex in JalbreakMe 3.0. If you want to update your firmware to latest iOS 4.3.4, simply connect your device to your computer and open up iTunes. iTunes will automatically pop-up a message that new update 4.3.4 is avaialble for your device. Just click Download and update and you are ready to go.
If you manually want to download this firmware update, here are the direct links: