stop mysql

/etc/init.d/mysql stop
mysqld_safe --skip-grant-tables &

login

mysql -u root

mysql

use mysql;
update user set password=PASSWORD("NEW_PASS") where User='root';
flush privileges;
quit

start mysql

/etc/init.d/mysql start

Links
http://www.cyberciti.biz/tips/recover-mysql-root-password.html
http://www.cyberciti.biz/faq/mysql-change-root-password/