Click on the «Activity» tab on the top menu of your dashboard and find your post by filtering via tags, sources or dates. Once you have found the post you want to edit, simply hover over it and the «Edit item» option will show up. Click on it and start updating. Remember to save all your changes once you're done.
To delete a post, proceed as above and click on the «Delete» tab that comes up when you hover on the article.
Warning : once deleted, you won't be able to recover your content.