My aunt-in-law recently bought a white Macbook through a friend and she asked me to change the laptop name from her friends' name to her own. It turns out that it's really a brand new Macbook, but in order to test the laptop, her friend typed in her name on the registration, which then became the name of the computer.
I knew I had changed my computer name previously but I forgot how to do it. The location is not very intuitive, but here's how to do it.
Open System Preferences and click the Sharing icon.
In the Sharing window, go to the box on the right of "Computer Name:" and enter your new computer name.
My first instinct was to look for this command under Hardware. To me, Sharing is more like tweaking permissions it does make sense after some figuring out (you share your computer by giving it a share name, which also becomes your computer name -- but if you're really not used to networking, then this won't be obvious). Anyway, I'm writing it down here because I had to change my Macbook name again and I forgot it again.