Update iOS software on iPhone
When you update the iOS software, your data and settings remain unchanged.
Note: Set up iPhone to back up automatically, or back up your device manually, before you update.
Update iPhone automatically
If you didn’t turn on automatic updates when you first set up iPhone, go to Settings 
> General > Software Update > Automatic Updates, then turn on Automatic Updates.
iPhone automatically installs updates wirelessly, and you’re notified before the update occurs. To see the currently installed version of iOS, go to Settings > General > Software Update.
Update iPhone manually
Go to Settings > General > Software Update > Automatic Updates, then turn off Automatic Updates.
At any time, you can check for and install software updates. Go to Settings > General > Software Update. The screen shows the currently installed version of iOS and whether an update is available.
Update using iTunes
Sync your device using iTunes.
iTunes checks for available software updates each time you sync.
See the Apple Support articles Update to iOS 12 and If you can’t update or restore your iPhone, iPad, or iPod touch.