How to install IOS 16

Have you tried the iPad passcode?

For the update, see the articles below.

Update your iOS device.  -

Get help with over-the-air iOS updates  -

Update – Can’t or can’t restore   -

Have you tried using iTunes on a computer or Finder if running Catalina or later to do the update?

Update software on iOS devices in iTunes on Mac. -

Phone, iPad, or iPod touch - Update with Finder.  *

iPhone, iPad, or iPod touch - Update with iTunes.  *

If you see an error when you update or restore your iPhone, iPad, or iPod    *

Updating iOS: common issues and solutions.  -

Maybe u have screen time setup? is it a 4 digit or a 6 digit pin its asking for? If it's a 4 pin and asking for screen time I believe you can reset your screen time password through your Apple ID. If you forgot your Screen Time passcode - Apple Support - Read this article if its your screen time password. (EDIT- My bad u don't even need to use your screen time password to update I was thinking of rest my bad)