How do I rename a user name from ‘tom’ to ‘jerry’ under Ubuntu Linux?

You need to use the usermod command. It can modify the system account files (such as /etc/passwd) to reflect the changes that are specified on the command line. The syntax is as follows:
usermod -l {new-login-name} {current-old-login-name}
In this example, rename login id from tom to jerry. Open a command-line terminal (select Applications > Accessories > Terminal), and then type the following command
$ sudo usermod -l jerry tom
To verify new changes, enter:
$ id jerry} else {