If you open the control panel for xampp/wamp, you should see a button that takes you to the config for phpmyadmin under the Config button for Apache.
There are fields in there for username and passwords.
To update the root username, I had to do the following:
1) Update the phpMyAdmin to use an auth_type of 'cookie' instead of config.
Login to phpMyAdmin using root and no password
Click on the Change Password
Enter your new password for root
It should log you out if you try to go anywhere else. Login using your new password. The config message should have disappeared now.