how to update 'update trigger fired record's column?

I’m new on this task.

I tried to update when I/F table insert after trigger. but it wasn’t work..

There is my trigger source like..

create trigger T_ATABLE_I 
after insert on ATABLE for each row
...
-- all sql queries done.
:new.IF_FLAG := 'Y'; -- it is I/F success flag on ATABLE.    
...

I heard after trigger can’t update :new..

But I must update this column when whole task just done.

conditions

1. ‘ATABLE’ table don’t have ID.
2. I must use ‘AFTER’ trigger because of some business role issue.

How could it be possible?