how do reset a forgotten pin on an ipad

You have to restore the device within iTunes. You want to use the same computer that you always sync with so that you can restore your app data and settings. You can restore with any other computer, but you will lose everything on the iPad.

You will probably have to use recovery mode

iPad: Unable to update or restore

If you mean the lock screen's unlock passcode:

IPhone, iPad, iPod touch: Wrong passcode results in red disabled screen