How can I edit existing ads in a campaign?

Editing Ads via is two step process. First of all you need to edit the ad template. If you wish you can create a new one too. For that kindly go to the ads dashboard and edit the template you wish.

If you wish to edit the destination url for the ads, you can do so by editing the url template. Destination url is the page the user lands after clicking on the ad. 

After editing the templates, kindly go to the campaign dashboard and select the campaigns to push the ads. You can even select search terms to push ads into.
Kindly check the following demo video to push ads.