Facebook allows you to edit your ad, until it is presented to your target audience. Once the ad has been used to advertise, you cannot edit it in any way. This is what we mean when we say "Ad:"
If your ad is running and you realize you need to make changes to it, make sure to deactivate the ad that is running.
Please see this article to learn how to deactivate your ad: How Do I Cancel My Ad?
If you meant, how do I edit my targeting, please review this article here: How do I edit my Targeting on my ad?