Changing Default Date Format

How do I change the Default Date Format to yyyy-mm-dd (NLS_DATE_FORMAT?).

See the alter session command at session level and NLS_DATE_FORMAT init paramter t database level

IT CAN ALL BE DONE BY CHANGING REGISTRY EDITOR
IF USING WINDOWS CLIENT

/HKEY__LOCAL_MACHINE/SOFTWARE/ORACLE
ADD STRING VALUE
NLS_DATE_FORMATE DD-MM-YYYY


THANKS
PITAMBER SONI