For starters, I moved your topic to Questions and Answers in the Welcome Mat -- this is because Ask An Expert is more for writing-related questions, and Questions and Answers is for questions about the site. No problem for getting the two mixed up, it's fine!
As for how to edit a story you've posted, if you look at the side of a work you've posted, under "Tools" there should be a white button that says "Edit Work"! It's right under your avatar and the buttons that say "Share on Facebook/Twitter".
It's not your fault ^^ You're new to the site, it might take you a while to get settled in, so don't feel bad about having to ask questions! Even things that might seem obvious in hindsight can be confusing when you're just starting out.
The idea that a poem was a made thing stayed with me, and I decided then that I wanted to be an artist, not just a diarist. So I put myself through a kind of apprenticeship in writing poetry, and I understood even then that my practice as a poet was deeply related to my reading. — Edward Hirsch