Tap Settings > System > System update > Check for update to check if updates are available.
When your phone notifies you that an update is available, just follow the instructions shown on your phone. If your phone is low on memory, you may need to move your apps, photos and other stuff to the memory card.
Warning: If you install a software update, you cannot use the device, even to make emergency calls, until the installation is completed and the device is restarted.
Before starting the update, connect a charger or make sure the device battery has enough power, and connect to Wi-Fi, as the update packages may use up a lot of mobile data.