Change my Oracle NLS Parameter Settings

Use the database procedure, dbms_session.set_nls(), which takes the name of the parameter plus its value

dbms_session.set_nls('nls_date_format','''dd/mm/yyyy''');

Note: The single quotes - the value must be exactly as it would be typed in.

An alter session command, eg:

alter session set nls_date_format = 'dd/mm/yyyy';