If you are a registered user when you log in to your account you will see a list of all the classified ads you've posted and you have the choice to edit or delete any ad.

Note that if you edit an ad that is already been approved by the administrator, the ad status reverts to Pending and it will become invisible to other users until the modifications you've made are approved by the administrator.

Classified ads posted as guest (i.e. without registering an user account) cannot be edited or deleted before their expiration date.