Try changing the following lines with new values
$cfg['Servers'][$i]['user'] = 'NEW_USERNAME';
$cfg['Servers'][$i]['password'] = 'NEW_PASSWORD';
Updated due to the absence of the above lines in the config file
Stop the MySQL server
sudo service mysql stop
Start mysqld
sudo mysqld --skip-grant-tables &
Login to MySQL as root
mysql -u root mysql
Change MYSECRET with your new root password
UPDATE user SET Password=PASSWORD('MYSECRET') WHERE User='root'; FLUSH PRIVILEGES; exit;
Kill mysqld
sudo pkill mysqld
Start mysql
sudo service mysql start
Login to phpmyadmin as root with your new password