As an administrator, you can edit any post that a user has created.
Navigate to the Menu and click on Discussions.
Locate the discussion you would like to edit - you can use the filters on the left-hand side, the search box, or just scroll down the page, depending on how many discussions there are.
Click on the discussion's title.
Click on the Edit link.
Edit the subject and/or body text in the popup window.
Click on Save.
The discussion will be amended.