And to what are we changing root on? The operating system, the RDBMS...? You cannot "replace" the 'root' user. It is builtin.
If Linux: look at the 'useradd' command to add a new user, the 'chgrp' command to change a user's group memebership, or better yet, consider using 'sudo' command to execute a command as 'root' without being logged in as root. This assumes that the user account that you are logged in as is part of the group that can execute the sudo command. These are just tips on where to begin your Google searches for more details.
If under MySQL: look at the 'grant' command at their online manual.
