How to Change NLS_CHARACTERSET in Oracle Database

 SQL*Plus: Release 12.2.0.1.0 Production on Wed Oct 3 19:00:03 2018

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Enter user-name: sys as sysdba
Enter password:

Connected to:
Oracle Database 12c Enterprise Edition Release 12.2.0.1.0 - 64bit Production

SQL>
SQL> select value$ from sys.props$ where name='NLS_CHARACTERSET';

VALUE$
--------------------------------------------------------------------------------
WE8MSWIN1252
SQL>
SQL>
SQL> shutdown immediate;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> select value$ from sys.props$ where name='NLS_CHARACTERSET';
select value$ from sys.props$ where name='NLS_CHARACTERSET'
*
ERROR at line 1:
ORA-01034: ORACLE not available
Process ID: 0
Session ID: 0 Serial number: 0


SQL> startup restrict
ORACLE instance started.

Total System Global Area 2566914048 bytes
Fixed Size                  8750112 bytes
Variable Size             687868896 bytes
Database Buffers         1862270976 bytes
Redo Buffers                8024064 bytes
Database mounted.
Database opened.
SQL> alter database character set internal_use AL32UTF8;

Database altered.

SQL> shutdown;
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL> startup;
ORACLE instance started.

Total System Global Area 2566914048 bytes
Fixed Size                  8750112 bytes
Variable Size             687868896 bytes
Database Buffers         1862270976 bytes
Redo Buffers                8024064 bytes
Database mounted.
Database opened.
SQL>
SQL>
SQL> select value$ from sys.props$ where name='NLS_CHARACTERSET';

VALUE$
--------------------------------------------------------------------------------
AL32UTF8