After the initial 12-hour editing window expires, you have to contact a site moderator. If you click on the main "Forums" tab at the top and scroll down, you'll see a long list of users. Moderators are shown in bold type. If you click on their name, it will take you to their profile where you will find an option to send them a message.
If it's a price change or something like that, you can announce it by posting to your own ad thread, which will bump it to the top and more people will see it.