First, open your device and navigate to Settings > All Settings > System backup and back up all your data. After that, connect to a Wi-Fi network and go to Settings > All Settings > About phone > Software Update. Tap Check for Updates/ Update option and wait several seconds. After your device find the new version, it will download and install it automatically. Also, you need to know that new updates may not always be available for all devices. They depend on your device manufacturer and your mobile service provider.