logout of gui, drop to shell (ctrl+alt+F1).
sudo adduser <newusername>
then add newusername to group sudo
exit
login to shell in that new name
sudo usermod -u <newUID> <usernametochange>
sudo adduser <newusername>
sudo usermod -u <newUID> <usernametochange>
fingers in many pies! a blog about: my nearest and dearest, following Jesus, John Deere, gasometers, nordic noir, Methodist ministry, rackmount servers, network security, pfSense, snap-on tools, linux and opensource, web publishing, multimedia in worship, John Mellencamp, video editing, Jim Wallis, Greenbelt, Modern Christian Art, civil engineering and so on and so forth...