You can easily edit a discussion you've already created.
In Course Navigation, click the Discussions link.
Click the title of the discussion.
Click the Edit button.
Edit the text in the Rich Content Editor. You can also edit other options within the discussion.
If you are editing an unpublished Discussion and want to publish it, click the Save & Publish button . If you are editing a published Discussion, the Save and Publish button will not appear. If the Discussion is already published or if you just wish to save it as a draft, click the Save button  to save the Discussion.