Before following these steps, you may wish to back up your important data.
For Mac OS X v10.5 or later
1. Enable the root user.
2. Log in as root.
3. Navigate to the /Users folder.
4. Select the Home folder with the short name you want to change, and rename it just like you would rename any folder. Keep in mind that the shortname must be all lowercase, with no spaces, and only contain letters.
5. Use the Users & Groups pane (Accounts pane in Mac OS X v10.6.8 or earlier) in System Preferences to create a new user with the Account name or Short Name that you used in the previous step.
6. Click OK when "A folder in the Users folder already has the name 'account name'. Would you like to use that folder as the Home folder for this user account?" appears. Note: This will correct the ownership of all files in the Home folder, and avoid permissions issues with the contents.
7. Choose Log Out from the Apple menu.
8. Log in as the newly created user. You should be able to access all of your original files (on the desktop, in Documents, and in the other folders of this Home).
9. After verifying that your data is as expected, you can delete the original user account via the Users & Groups pane (Accounts pane in Mac OS X v10.6.8 or earlier).
10. Disable the root user
http://support.apple.com/kb/HT1428
