Please follow the step-by-step guide to update the iOS version on your iPad:
Step 1: Connect the iPad to a power source and then connect the iPad to a stable and working Internet connection using Wi-Fi.
Step 2: Click on Settings > General > Software Update.
Step 3: Tap on 'Download and Install'
Note - If a message asks to temporarily remove apps because iOS needs more space for the update, tap Continue or Cancel. Later, iOS will reinstall apps that it removed.
Step 4: To update now, tap Install. Or you can tap Later and choose Install Tonight or Remind Me Later. If you tap Install Tonight, just connect your iPad to a power source before you go to sleep. Your iPad will be updated automatically overnight.
If asked, enter your password.