Thank you for the details of the issue you are experiencing when trying to perform an update. I recommend trying to update using iTunes, and as always, it is a good idea to make a backup first.
Thank you for using Apple Support Communities.
The passcode is the four digit number that you enter after an Autolock on your device or when you first turn it on. If you can't remember that code you'll have to restore your iPad before you can do the update.