Connect to the Internet (preferably using Wi-Fi connection). Go to menu "Settings". In most cases it is displayed by using the icon of a gear. Go to "System data". Some of the gadgets he called the "General settings" or "About the system". You will see a menu with various settings.
Find the item called "Automatic updates". Examine whether these settings. If not, turn them on. If you don't want Android to update automatically, click on the button "Update". The operating system will be updated to the latest available stable version.
Keep in mind that the system files weigh quite a lot, so you will not be able to use the phone for several minutes (or even hours). It is recommended to put the updates at night, because you don't have to wait until gadget will change. This is the easiest way to upgrade Android on the phone.
If the previous method does not work for you, go to the app Play Market. There in the update section you can find the right version for your phone. If not, then download one of the apps that allows you to quickly search for and install the latest version of operating systems.
You can also go to the official website of "Android" and find the update for your phone there. Download the file first to the computer then transfer to phone and run. The principle of operation is no different from installing a normal application.