Update iOS software on iPad
When you update the iOS software, your data and settings remain unchanged.
Note: Set up iPad to back up automatically, or back up your device manually, before you update.
Update iPad automatically
If you didn’t turn on automatic updates when you first set up iPad, go to Settings 
> General > Software Update > Automatic Updates, then turn on Automatic Updates.
iPad 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 iPad 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 iPad 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.