As you may already know, the internal storage of a date column doesn't change, regardless of how the date is displayed. Therefore the question boils down to modifying the format for display purposes.
1. To change the format within a single sql statement, specified the desired format:
select to_char(sysdate,'dd-mm-yyyy') from dual;
2. To change the format for the duration of a session
alter session set nls_date_format='dd-mm-yyyy';
3. To change the format for all users, set the value of nls_date_format in the init.ora file, then shutdown and restart the instance.
nls_date_format = 'dd-mm-yyyy'