If a message says that an update is available, tap Install Now. You can also follow these steps:
Plug your device into power and connect to the Internet with Wi-Fi.
Go to Settings > General, then tap Software Update.
Tap Download and Install. If a message asks to temporarily remove apps because the software needs more space for the update, tap Continue or Cancel. Later, iOS or iPadOS will reinstall apps that it removed. If you tap Cancel, learn what to do next.
To update now, tap Install. Or you can tap Later and choose Install Tonight or Remind Me Later. If you tap Install Tonight, just plug your device into power before you go to sleep. Your device will update automatically overnight.
If asked, enter your passcode. If you don’t know your passcode, learn what to do.