Before you update your iPhone iOS, make sure you have backed up all your data to iTunes or iCloud just in case you have to re-install it after the update. Typically, you won’t lose any data, but it’s better to be safe than sorry as every once in awhile issues can happen. Also, make sure your iPhone is fully charged before you try to update your iOS wirelessly. After you have backed up your data and have made sure your iPhone is charged, double check to make sure you are connected to Wi-Fi so you don’t have to pay additional data fees.

When your iPhone is connected to Wi-Fi, go to Settings, General and then Software Update. Your iPhone will automatically check to see if any updates are available and will start to download right away if they are. When the download is complete, tap on “Install” and your iOS will update. If your iPhone isn’t updating by itself, you may have to update the iOS through iTunes. Make sure you have the most up to date version of iTunes and then connect your iPhone. Choose your iPhone under Devices and click on “Check for Update”.

Source Apple Support