This is the first time ever i decided to take part in a comp of...

This is the first time ever i decided to take part in a comp of this kind
but i have encountered a little problem:

the initial combination of stocks i tried to enter has already been submitted
by others so i tried different ones but each time there was a rejection
as each combination seems to have already been submitted,
so i gave up not seeing the point in just entering stocks of top of my head
until the system accepts something.

Is there anything i m doing wrong or something i am missing
and /or a way to change the selected stocks entered
-if i manage to do so successfully- if later on i change my mind?