MySql 从8.0开始，修改密码有了变化，在user表加了字段authentication_string，修改密码前先检查authentication_string是否为空:

1、如果不为空,则先置空

USE mysql;
UPDATE user SET authentication_string='' WHERE user='root';
ALTER USER 'root'@'localhost' IDENTIFIED BY 'root';

2、如果为空，直接修改

ALTER USER 'root'@'localhost' IDENTIFIED BY 'root';

如果出现如下错误:
ERROR 1290 (HY000): The MySQL server is running with the –skip-grant-tables option so it cannot execute this statement

mysql> GRANT ALL PRIVILEGES ON *.* TO IDENTIFIED BY '123' WITH GRANT OPTION;

也可以先执行，再修改

FLUSH PRIVILEGES;

然后再执行

ALTER USER 'root'@'localhost' IDENTIFIED BY 'root';