Users, modifying

There are many ways to modify existing users on your system, depending on what you want to do; the easiest is to edit the relevant files manually.

To change a user's shell, use chsh.

To change a user's name, edit /etc/passwd.

To change a user's login name (account name,) you can also edit /etc/passwd, but the easy (and safe!) way to do it is to add a new user and move the user's files over to the new user and chmod and chown everything to that new user.