There are a couple of places on your Mac where your name – or some other name – might appear: the account name and Home folder. The name that appears is based on the Full Name you gave when you (or the administrator) first set up the account.
But what if you want to change your username as it appears in these locations? It’s possible that you have since married and now have a new name (or you just divorced). Maybe IT misspelt your name when they set up your Mac. Perhaps your initial and surname spell something offensive, or it might be time to change what was once an amusing username for something more formal. Maybe your current short name isn’t really short and is a pain to type every time you’re prompted for your username. Perhaps you’ve got two or more Macs and you want to use the same short username on all of them. Or maybe you inherited a Mac and would prefer to just change the name of the existing account rather than create a new one.
Read more at Macworld.com