If you’d like to change your password in a Linux system, there is a terminal command could be useful, passwd.
For changing your own passwd, type
and follow the instructions.
If you have installed Ubuntu (or a similar distribution), the root account has not a defined password – ’cause the administrator account is not the root one -, i f you want to set a password for the root, use the following command
$ sudo passwd
and after given the administrator password, type the new root password, and re-type for confirming (just follow the instructions).
If you are the administrator and wish to change the password of the guess user, type
$ sudo passwd guess
and once more follow the instructions.