how to remove lubuntu and install ubuntu

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http://askubuntu.com – I recently purchased a Thinkpad R40 with Lubuntu on it but don't have the install cd's. The person I got it from doesn't know the administrator password so I can't access any changes. I went to the root screen thinking maybe I could change the password that way but it keeps telling me Give root password for maintenance (or type Control-D to continue). I was hoping to uninstall Lubuntu and install Ubuntu but don't know the steps for this. Can anyone please help me? I am new to Linux and don't want to mess anything up. (HowTos)