Type "man passwd" to see how the password command works. But, essentially, login as root and type "passwd".
A caveat: Normally, we never actually login as root. When we need to do things as root, we can either use "sudo" followed by the command, or just "su" or "su -" to login to root from a terminal. Being root as a GUI user can cause you to get into all kinds of bad habits that are best avoided by logging in as usual and then becoming root in a terminal.