# 1
mysql -u root
SET PASSWORD FOR 'root'@'localhost' = PASSWORD('[new password]');

# 2
mysqladmin -u root password "[new password]";
mysqladmin -u root password "[old password]" "[new password]";

# 3
mysql -u root
use mysql;
UPDATE user SET Password = PASSWORD('[new password]') WHERE user = 'root';
FLUSH PRIVILEGES;

# 4
mysqld_safe --skip-grant-tables # modify my.cnf or my.ini
mysql -u root # login
use mysql;
UPDATE user SET password=PASSWORD('[new password]') WHERE user='root';
FLUSH PRIVILEGES;