Change character set in oracle database - AL32UTF8

Login to the database and execute below commands :

SQL> SELECT VALUE FROM NLS_DATABASE_PARAMETERS WHERE PARAMETER='NLS_NCHAR_CHARACTERSET';

VALUE
----------------------------------------
AL16UTF16


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

SQL> startup Restrict;
ORACLE instance started.

Total System Global Area  430075904 bytes
Fixed Size                  2176448 bytes
Variable Size             343935552 bytes
Database Buffers           79691776 bytes
Redo Buffers                4272128 bytes
Database mounted.
Database opened.

SQL> ALTER DATABASE CHARACTER SET INTERNAL_USE AL32UTF8;

Database altered.

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

SQL> startup;
ORACLE instance started.

Total System Global Area  430075904 bytes
Fixed Size                  2176448 bytes
Variable Size             343935552 bytes
Database Buffers           79691776 bytes
Redo Buffers                4272128 bytes
Database mounted.
Database opened.

SQL> SELECT VALUE FROM NLS_DATABASE_PARAMETERS WHERE PARAMETER='NLS_NCHAR_CHARACTERSET';

VALUE
----------------------------------------
AL32UTF8