$>mysqld_safe --skip-grant-tables
$>mysql -uroot
$>ALTER USER 'root'@'localhost' IDENTIFIED BY 'MrSi3u@#!';
----------or-----------
$>UPDATE mysql.user SET authentication_string = PASSWORD('MrSi3u@#!'), password_expired = 'N' WHERE User = 'root' AND Host = 'localhost';
--------MySQL 5.7.5 and earlier
$>SET PASSWORD FOR 'root'@'localhost' = PASSWORD('MrSi3u@#!');
$>FLUSH PRIVILEGES;