You can edit posts. I'll post this first, first - save - then come back and take a picture of the button you use to edit - then post/update the picture of that button.
(saving here - before edit)

EDIT: I pressed the edit button and added this line and the picture below - showing the message before I edited it.

NOTE: you may not be able to delete the 1st message in a thread. I'm not sure. Since I see some have opted to edit their post and replace all the content with a dot ("." ) or a message like "DELETED". Use delete if you can. Although it would be "bad form" to delete the 1st post of a thread you created if there are useful answers others have contributed.

On the other hand - if you just want to delete your response within a thread (not deleting the entire thread) - there's no harm or foul in deleting your own responses.

Thanks.