# Login as root - kill all your user sessions
# You could also ssh-in as root

usermod -l [newname] [oldname]
groupmod -n [newname] [oldname]
usermod -d /home/[newname] -m [newname]

# Optional
# usermod -c "[full name (new)]" [newname]

# Verify
id [newname]

# Add a symbolic link to your old user for programs that use absolute paths to
# still work
ln -s /home/[newname] /home/[oldname]