mysql5.7：

mysql> use mysql;

mysql> update user set password=password('新密码') where user='用户名';

or 

mysql> update mysql.user set authentication_string=password('新密码') where user='用户名';

mysql> flush privileges;  刷新

mysql8

mysql> use mysql;

mysql> ALTER USER '用户名'@'localhost' IDENTIFIED WITH mysql_native_password BY '新密码';

mysql> flush privileges;刷新