Following are the set of commands to be run to reset root user password:
# 1. Stop mysql process
> sudo service mysqld stop
# 2. Change user to mysql & run mysql process using --skip-grant-tables option
> sudo su - mysql
> /usr/sbin/mysqld --skip-grant-tables --skip-networking &
# 3. Login in mysql shell as root user & set the new password.
> mysql -u root
> flush privileges
> SET PASSWORD FOR root@'localhost' = PASSWORD('newpassword');
> quit
exit
# 4. Restart mysql process
> sudo service mysqld start
# 5. Now you can login using new password
> mysql -uroot -p