1、用sysdba账号登入数据库，然后查询到要更改的用户信息：
SELECT user#,name FROM user$;
2、更改用户名并提交：
update user$ set name='demokygs' where user#=111;
COMMIT;
3、强制刷新：
ALTER SYSTEM CHECKPOINT;
ALTER SYSTEM FLUSH SHARED_POOL;
4、更新用户的密码：
ALTER USER PORTAL IDENTIFIED BY "test23";
alter system set "_enable_rename_user"=true scope=spfile;
startup restrict force;
alter user kygs rename to demokygs identified by "test23";
19.10以后禁止以上更改，只能通过expdp、impdp修改
