HELPPPPP!!! with OS X Mavericks Update

A new IMac computerr at Bestbuy today, I tried to download the "OS X Mavericks v10.9.2 update" but it has the username of an employee of Bestbuy. I have no idea how to erase that username. I'm asking for help because what i don't understand is that I'm signed in but when I try to download Mavericks it doesn't have my username in. Does anyone have suggestions?