Here are the instructions from Apple on changing user short name or home directory name. I've used this on a couple of used MacBooks I've bought on eBay. And remember to go into System Preferences>Sharing and change the Computer Name in there.
Note that while it is possible to change the name of a home directory, it's not recommended that you do so since such a change can cause unexpected and in some cases very undesirable issues. It would be best to create a new user account for yourself, log into the system using that account, copy over any documents from the existing account you wish to keep, and then delete the old account.