TO change a users name is as simple as using the following command:
usermod -l new-name old-name
In the above example the new-name represents the name that you would like to change the users name to and the old-name is what the name was before.
To change a users UID or user ID you can use the following command
usermod -u UID username
In this example the UID is replaced with the value that you want the new UID to be, 501 for example and the username is the users username.