for your session, you can use :
ALTER SESSION SET NLS_DATE_FORMAT='...'
where ... is the format you want, for example
HH24:MI:SS for hours on a 24h format, minuts, seconds : 18:31:43
DD/MM/YYYY gives day/month/year : 25/07/2001
if you want to change for the whole instance, add NLS_DATE_FORMAT='...' in your init.ora file