I want to sell my iPad 2 so I can upgrade to the new one .how do I reinstate . To what it was when I received it

Settings > General > Reset > Erase All Content and Settings

https://support.apple.com/kb/ht2110

Make sure you pick this option and not one of the others. It will securely wipe the iPad, leaving it empty and ready for a new user to set up.

NB: Ensure that you have performed a backup beforehand, via iTunes and/or iCloud, so you can then restore the backup to your new iPad once you buy it.

Thank you ,