I have had to reset a MySQL root password a few times recently so I should probably write it down.
First, stop the currently running MySQL database daemon
sudo /etc/init.d/mysql stop
Next, start MySQL in safe mode without grant tables, which store the passwords
sudo mysqld_safe --skip-grant-tables
Now login to MySQL as root with no password. You may need a new terminal since MySQL is probably still running from the above command and you don't want to kill that yet.
mysql -u root
Update the root password
update user set Password=PASSWORD('mypwd') where user='root';
Logout, kill mysqld_safe with ctrl-c and restart MySQL.
That should be it.