In the upper right corner hover over your user name. Click "my listings" and edit the listing.

Hello...You will have to pull up your CarGurus account, or you can pull the account up from the email they originally sent to you when you first posted your listing. When you pull up your account, and click on "my listing", every listing you have posted should pull up. Then click on the listing you what to edit, then at the bottom of your pictures, you will see "edit ad", then click it.