purchased s/hand iMac still got old name

  Dells 12:15 03 Jan 2015
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Hi all I have just purchased a s/hand 2009 iMac with Yosemite and although I have changed name in sharing and everywhere else I could find in terminal and iTunes it is still saying Colin's iMac instead of my name could anyone tell me how I could change this.

Just a quick update on my problem although I have ordered a snow leopard from apple I thought I would try from a bootable Yosemite so I did a google search on how to make one on a flash drive booted from said flash drive and erased SSD and installed a clean install of Yosemite but in terminal it is still saying Colin's iMac Derek$ .So I must be missing something I can't understand how installing snow leopard will make any difference could somebody please explain why it will make any difference .

This is my problem and what i have tried i have asked this on apple forum no joy so far so i thought i would ask here Thanks Dell

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