You can make changes to a Classified at any time.
Simply go to your dashboard and navigate to Classifieds in the menu on the left. Use the drop down to select Classifieds.
Then click on the blue edit button next to your Classified listing.
This will open your Classified listing details where you can make changes to the applicable field(s) and proceed through the steps by clicking Next.
When you have finished making changes, click Submit at the bottom of the final tab (Media) and your Classified will be updated.