Log into MySQL, select database mysql and execute an update statement with the new password.
It is important to set the user name (root) at the where clause of the statement.
The same method can be used to change password of any other user. The changes will take effect after restarting MySQL.
mysql> use mysql;
mysql> update user set password=PASSWORD('mypassword') where User='root';
Query OK, 2 rows affected (0.00 sec)
Rows matched: 2 Changed: 2 Warnings: 0
After the change of the root password phpMyAdmin will no longer have access to the database.
To fix this issue locate the installation directory of the phpMyAdmin and open file config.inc.php.
Navigate to the line that starts with $cfg['Servers'][$i]['password'] and modify it:
$cfg['Servers'][$i]['password'] = 'mypassword';