UPDATE mysql.user SET Password=PASSWORD('MyNewPass') WHERE User='root';
FLUSH PRIVILEGES;

mysql> SET PASSWORD for 'root'@'localhost' = password('enteryourpassword');
mysql> SET PASSWORD for 'root'@'127.0.0.1' = password('enteryourpassword');
mysql> SET PASSWORD for 'root'@'::1' = password('enteryourpassword');