Extract and edit page's iframe

Hello I'm trying to select certain elements from an iFrame but I don't know how to reach the code containing the iframe.

For example in recaptcha (http://ift.tt/1AcNpMS) how to reach the iframe and edit the code just like we can do manually if we click on the checkbox, right click on the image selection window and click on inspect element.

I want to reach the same code that we can reach to by inspect element and edit it though I cant seem to figure out how to do it.