To edit the Targeting you are using on your ad, start by going here: https://www.facebook.com/ads/manage.
To open the ad, you click on the Campaign name and keep clicking the name until you can see the ad below it.
Then Click the Gear to open the targeting, and then click edit on the right above the targeting.
See the image below for what to expect on your screen:
Is this too small to see? Click this link to see a larger version of the image: http://screencast.com/t/RbbEIGzaAHw