The Survivors Stories forum is set up so that men can tell their stories without fear or consequence. For that reason the forum is set not to accept replies or edits.
For a bigger change, sending the updated version via PM is a good idea, but if it's just the trigger warnings you want added, you don't have to go to the trouble of copy/pasting the whole thing. You can just ask one of the forum mods or admins to add the warnings where you want them.
By the way, thank you for being considerate about the triggers.