To change the main cover photo of a listing, go to the listing under Active Ads, and then hover the mouse over the cover photo until the tooltip says Change cover photo and then click on the photo:
This will take you to a photo carousel where you can pick the new cover photo:
To change photos, either click on the photo at the bottom, or click the arrows to the left and right of the page to cycle through them. When you have selected the photo you want, click Make cover photo:
Now the cover photo has been changed!
Note that the cover photo just refers to the main display photo that renters see first. If you want to change the order of the photos for a listing or add or remove photos entirely, you'll need to go to the Edit page under the Properties section.