Using The System Update Tool
1. Open the Settings menu on your device. You can open this through the Settings app on your Home Screen or in your App Drawer, or by pressing your device's Menu button and selecting Settings.
It is recommended that you connect your phone to a wireless network when updating, as update files can be quite large and put a big dent in your mobile data.
2. Scroll down to "About...". It may say "About phone" or "About device". This is usually located at the bottom of your Settings menu. Tap it to open it.
3. Tap the "Update..." option. It may be labeled "Software update" or "System updates". This is typically located at the top of the "About phone" menu.
If you don't have a "System updates" option, your phone does not support updating over the air. You will need to visit the support page for your phone and download any updates directly from the manufacturer. This is only an issue for older Android phones.
4. Scan for new updates. Tap "Updates" or "Check for system updates" Your phone will then scan to see if there are updates available.
Just because a new version of Android is out doesn't mean it will be available for your phone. It is up to the manufacturer and the carrier to make the update available for your device, and there is a good chance that the update will never be available, especially if your device is older.
If you really want to update to a newer version of Android and your device isn't allowing it, you may be able to root your phone and install a custom ROM.
5. Tap "Download" to begin downloading an available update. Make sure that you are connected to a wireless network, as these updates can be quite large.
6. Tap "Install" or "Install now" to begin the installation after the download is complete. If you accidentally closed out of the download confirmation, you can start the installation by returning the the Update tool in the About Phone section.