By any Ad you can see three dots where you can see "Edit Ad" menu.
In the list where you can see all your Ads, active, inactive and past, you can edit each of them by clicking on "Edit Ad" button. You can edit active, inactive and past Ads.
On the existing Ad you can only change an Ad cover photo, Ad description, for who your Ad is intended to and Ad details.
Read How to prolong my existing Ad or Where can I see all my Ads for more information.
