Read-only to Read-write access?

Hi Oracle,

I am building a new database by all datafiles of a standby database.

My new database has been set up. I can login it as any users. However, it was on read-only access.

How can I change it as read-write access?

My actions.

as sys user

--shutdown immediate

--startup mount

--alter database open read write;

I got, "ORA-16003: standby database is restricted to read-only access"

--select open_mode,database_role from v$database;

OPEN_MODE  DATABASE ROLE
==============  =================
READ ONLY  PHYSICAL STANDBY

Please advise

Thank you

Liz