Please note that this depends on the version of Plesk you are using.
In order to log in to the PHPMyAdmin control panel as root rather than a specific user you can do the following from within your VPS\VM Plesk control panel:
Log into the Plesk control panel and then go to:
Server >> Database Servers
To login to PHPMyAdmin you can click the phpmyadmin icon to the far right of the MySQL database server.