In case you have forgotten your password, go to the administrative login screen and you will see a field named "Forgot Password?: Enter your email:". There you will have to enter the email address associated with the administrative username and an email with the password will be sent to that address.
In case you do not remember the email either, you can change the password manually from the MySQL database. The easiest way to achieve this is to use the phpMyAdmin tool. Go to your cPanel and click on phpMyAdmin.
Then select the database used by the phpList from the drop-down on the left side, click on the table named "phplist_admin" then click "Browse". You will see a line with different fields with information regarding the administrative account, click on the pencil icon and you will be able to edit the field which contains the password. This way the administrative password can be reset or changed.