how do i download ios 9.3

Based on your current iOS build, the process might vary. On your device, go to Settings > General > Software Update, there wait for the page to load then proceed to download and install iOS 9.3.3. On iTunes, simply go to your device tab, then select Check for update. For more info, please check out this page by Apple here. Also note that iOS 9 is only supported on certain devices, check your compatibility here. (scroll to the bottom of page to see device compatibility)