Editing your ad is easy. It's a lot like creating the ad.
First, make sure you are logged in
Second, go to your ad
Third, click the "Edit" tab at the top of the ad.

There you will see

On this page you can edit the text of the ad, your phone number, or any other information you set when you created the ad.
You can also "Reset Ad Expiration" which allows your ad to stay up for another week.
You can also add pictures to your ad by clicking the "Images" tab (again, only visible if you are logged in). Below you can see the "Images" page,

The "Description" block allows you to describe the picture you are uploading.
The "List" button indicates whether or not you want an image to be visible. Yes means you DO want the image to be visible.
The "Browse" button allows you to look for the image on your computer.
Images must be less than 1MB (MegaByte)
You may upload 3 images to each of your ads.
Finally, hit the "Upload" button to upload the pictures to your advertisement.