There are two ways to update the operating system on the iPhone. First phone update via wifi on the device itself. For this you will need a source of Internet Wi-Fi, charge by phone more than 60% or a charger connected to the phone. To update go to the menu item "Settings", it enter in the submenu of "Basic" and then "software Update".
In the opened window you will be prompted to update, accept it and wait for download and update the phone's firmware. It is important not to turn off the Internet and do not enable airplane mode during the upgrade, otherwise the download of iOS will cease and the phone will not be updated. When complete, the phone will start its installation. Don't be scared of the black screen of your smartphone, which appeared on the company sign - bitten Apple, a loading bar under it indicate the process of installation. After installation, the phone will turn on and you will be able to enjoy all the benefits of the new operating system.
The second way to install new software through iTunes. It will be useful for those who cannot upgrade the firmware via Wi-Fi. Download iTunes to manage the iPhone from the computer from the official Apple website. Connect your phone to computer via USB cable, run iTunes and open the iPhone. The corresponding button should appear in the upper right corner of the screen. When you open the device, click "Update" and wait for the start file download. Do not disconnect the phone from the computer, while iOS will not install and also follow the continuity of Internet connection.