Alter database character set

SHUTDOWN IMMEDIATE;
STARTUP MOUNT; 
ALTER SYSTEM ENABLE RESTRICTED SESSION;
ALTER SYSTEM SET JOB_QUEUE_PROCESSES=0;
ALTER SYSTEM SET AQ_TM_PROCESSES=0;
ALTER DATABASE OPEN;
ALTER DATABASE CHARACTER SET INTERNAL_USE WE8MSWIN1252;  -- you can try
        -- without INTERNAL_USE first.
ALTER DATABASE NATIONAL CHARACTER SET UTF8;
SHUTDOWN IMMEDIATE;
Note that INTERNAL_USE is undocumented Oracle feature and Oracle does not
suggest to use it. However on Metalink you can find several articles that
references to this option. E.g. Note:225938.1. ALTER DATABASE does not
convert characters - it just changes database character set definition.

Test on non-production environment before doing the change on production.
And make a backup of database before altering character set.

try this
ALTER DATABASE [db_name] NATIONAL CHARACTER SET new_NCHAR_character_set;