1.Reboot your computer and login as  root

2.Open a terminal and Use blow command to change your username

usermod  -l newname oldname

3.(Optional)Now you already changed your username,but your home folder name still are “/home/oldname”,if you also want to change /home/oldname to /home/newname just run below command

usermod -m -d /home/newname newname

4.Note:Of course you can combine step 2 and step 3 using below command

usermod -m -d /home/newname -l newname oldname