If the process has stalled, you will need to restart the iPad. Connect your iPad to its Power Adapter - and perform a forced-restart:
Force restart iPad (models with Face ID or with Touch ID in the top button)
If your iPad doesn’t have the Home button, do the following:
- Press and quickly release the volume button nearest to the top button.
- Press and quickly release the volume button farthest from the top button.
- Press and hold the top button.
- When the Apple logo appears, release the top button.
Force restart iPad (models with the Home button)
Press and hold the top button and the Home button at the same time. When the Apple logo appears, release both buttons.
After restart the iPad, resume the update process. You would be well advised to keep the iPad connected to its Power Adapter at least until the update has successfully concluded.
Maintaining connection to an external power sources serves two purposes:
1) Your iPad will keep its WiFi connection active throughout the process.
2) Ensures that adequate power is always available from two sources of power (external + battery) - preventing a potential power interruption from corrupting the update/installation process.