Root User Change

Hello. I have windows 8 with the upgrade. I bought this computer from someone who reinstalled the operating system so he could sell it. The root user name is John Doe. I adjusted settings and tried to rename with my name however the ROOT user is still John Doe. I've heard I can create a new account and "move" all my files to my new account, thus essentially renaming the computer. I've heard all that changes is the "pointer". I'm fairly tech savvy so this is a bit embarassing, but this is something I have no experience with. Could someone please tell me exactly which files I need to move so I can delete the "John Doe" user? I'm a bit nervous about messing things up. Please give a brief walk-through if you would.

Thanks!
:)