First, BACKUP!!! (in case you mess something up)
Here are the steps: (you should disable auto-log in and file vault before continuing, then LOG OUT of the account you want to modify -DON'T just use fast user switching. LOG IN TO A DIFFERENT ACCOUNT FROM THE ONE YOU ARE MODIFYING)
1. Click >System Preferences...>Accounts.
2. Unlock the lock, if necessary.
3. Control click your user's name, and click "Advanced options".
4. In the "advanced" pane, highlight the "Short name" field, and replace it with the desired new short name.
5. In the Home Directory field, change /Users/oldusername to /Users/newusername, where oldusername is your original short username and newusername is your new short username. Write down the original and new paths.
6. Click OK and quit system preferences.
7. Open /Applications/Utilities/Terminal
8. Copy the blue text, and press enter, but REPLACE '/Users/oldusername' and '/Users/newusername' with the correct paths, which you wrote down in step 5.:
sudo mv /Users/oldusername /Users/newusername
9. Press enter, and type your password. When the prompt comes back, restart your mac.
Good luck!