首先my.ini 
添加

#跳过密码

skip-grant-tables

重启mysql服务

然后执行修改密码会报错，

mysql> ALTER USER 'root'@'localhost' IDENTIFIED BY '123456';
1290 - The MySQL server is running with the --skip-grant-tables option so it cannot execute this statement

先要刷新权限就行了
mysql> flush privileges;
Query OK, 0 rows affected (0.00 sec)

mysql> ALTER USER 'root'@'localhost' IDENTIFIED BY '123456';
Query OK, 0 rows affected (0.00 sec)

再注释掉跳过密码

重起mysql服务，输入密码ok