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March 11 was the day that Apple had officially announced the release of iOS 4.3. However I have just read news that version 4.3 has been released today.
To update your iOS device(s) to version 4.3, open iTunes on your desktop computer, attach your iOS device (iPad/iPhone), and ensure it is selected in the Devices section, and click the Check for Update button (found on the Summary tab) in iTunes. iTunes will give you status updates and instructions to follow during the upgrade process.