Software update is available but asking for passcode which do not know. How i would know passcode?

The passcode you need to enter is the passcode you use when the iPad is first turned on or awakened from sleep after an Autolock.

No such thing as a "default" passcode.

Any passcodes being asked for on a iDevice you have to manually set. So if you don't know it you need to find someone who does.

The Passcode is probably your Apple ID password.

Thanks