The following describes in a multitenant container database(CDB) open  pluggable database (PDB) as read write mode and trun off the read only mode.

ALTER PLUGGABLE DATABASE

The ALTER PLUGGABLE DATABASE command can be used from the CDB or the PDB.

ALTER PLUGGABLE DATABASE <pdd-name-clause>  OPEN READ WRITE [RESTRICTED] [FORCE];

ALTER PLUGGABLE DATABASE <pdd-name-clause>  OPEN READ ONLY [RESTRICTED] [FORCE];

ALTER PLUGGABLE DATABASE <pdd-name-clause>  OPEN UPGRADE [RESTRICTED];

ALTER PLUGGABLE DATABASE <pdd-name-clause>  CLOSE [IMMEDIATE];

[oracle@migrator admin]$ sqlplus / as sysdba

SQL*Plus: Release 12.1.0.1.0 Production on Thu Aug 6 14:12:58 2015

Copyright (c) 1982, 2013, Oracle.  All rights reserved.

Connected to:

Oracle Database 12c Enterprise Edition Release 12.1.0.1.0 - 64bit Production

With the Partitioning, OLAP, Advanced Analytics and Real Application Testing options

SQL> select con_id,dbid,NAME,OPEN_MODE from v$pdbs;

    CON_ID       DBID NAME                           OPEN_MODE

---------- ---------- ------------------------------ ----------

         2 4129503604 PDB$SEED                       READ ONLY

SQL> SELECT pdb_name, status FROM   dba_pdbs ORDER BY pdb_name;

PDB_NAME                       STATUS

---------------------- -----------------------

PDB$SEED                      NORMAL

SQL> alter pluggable database pdb$seed open read write;

alter pluggable database pdb$seed open read write

                         *

ERROR at line 1:

ORA-65017: seed pluggable database may not be dropped or altered

SQL> select cdb from v$database;

CDB

---

YES

SQL> alter session set "_oracle_script"=TRUE;

Session altered.

SQL> alter pluggable database pdb$seed close immediate instances=all;

Pluggable database altered.

SQL>  select open_mode from v$database;

OPEN_MODE
--------------------
MOUNTED
 SQL> alter pluggable database pdb$seed OPEN READ WRITE;

Pluggable database altered.

SQL> show con_name;

CON_NAME
------------------------------
PDB$SEED

SQL> select open_mode from v$database;

OPEN_MODE
--------------------
READ WRITE

 SQL> alter pluggable database pdb$seed close immediate instances=all;

Pluggable database altered.

SQL> alter pluggable database pdb$seed OPEN READ ONLY;
Pluggable database altered.

SQL> show con_name;

CON_NAME
------------------------------
PDB$SEED

SQL> select open_mode from v$database;

OPEN_MODE
--------------------
READ ONLY

SQL> alter session set "_oracle_script"=FALSE;
Session altered.

That’s all, Hope this helps.