Here are simple manual about resetting forgotten root password on MySQL. This actions cannot be done without root access to computer/server, so you will need root password for this action. On this text we will reset password on Ubuntu 14.04 server.

# Stop MySQL server. While it's stopped all sites which are connected to dataase will be inaccessible
# If you're use Ubuntu run
sudo /etc/init.d/mysql stop

# If you use CentOS run
sudo /etc/init.d/mysqld stop

# Go into MySQL safe mode
sudo mysqld_safe --skip-grant-tables &
mysql -uroot
use mysql;
update user set password=PASSWORD("yournewpassword") where User='root';
flush privileges;

# (re)Start MySQL server
# Ubuntu
sudo /etc/init.d/mysql restart

# CentOS
sudo /etc/init.d/mysqld start

# Try to login to MySQL as root
mysql -u root -p