changing computer name - not short name, not account name

When I bought my iMac I had to give the Apple store my legal first name because that's what's on my credit card. When I revved the iMac up, I discovered that a helpful Apple technician had named her "HorribleFirstName Harper's iMac", which now shows up in Finder and in the computer name screensaver.

Somebody PLEASE tell me I can change this???

Thx.