Changing a users name:

sudo usermod -l new-username old-username

To change the users directory name do the following.

 sudo usermod -d /home/new-username

You might need to log in as root or another user to successfully execute the commands.

Adding a user:

sudo useradd -m newuser

The -m option create the new users home directory

Then to activate the user we need to setup a password.

sudo passwd newuser

Enter and reenter the password and your done.

If you need to add the user to a certain group(s) (sudo?) you can do that with usermod

sudo usermod -G sudo,othergroups newuser

Deleting a user:

sudo userdel -r username

The “-r” option deletes the users mail and home directory.  If you wanted to keep them then just discard that option.