I am having Column value as 'YYYYDDMM' and need to convert into 'DD/MM/YYYY' and store the data.
OK, I'm assuming that both the input and the output are strings. So you just need to nest a couple of functions - one to take the input into a date, and one to take it back to a string
SQL> create table t ( x varchar2(10));
Table created.
SQL>
SQL> insert into t values ('20180327');
1 row created.
SQL>
SQL> select
2 to_char(
3 to_date(x,'yyyymmdd'),'dd/mm/yyyy') from t;
TO_CHAR(TO
----------
27/03/2018
By the way, check out Chris's video why you don't want to store dates as strings...its bad
So ideally, you just want to store your dates and dates - and only format them a strings when you need to present them to the user (on screen or in reports etc)