1. Click on the "Comments" link on the left-hand menu. This will take you to the Comments list page.
2. Hover the mouse over the comment you wish to edit, to show the actions menu, and then click the "Reply" link.
3. A reply box will then be shown below the comment, where you can enter your reply.
4. Enter your comment and click the "Reply" button on the right-hand side of the reply box.
NOTE: When you leave a comment in this way, the account you are logged in with will be used for the name, etc.