Clicking on Edit, or in fact the media file name or thumbnail, will take you to the Edit Media screen, which looks like this:
At the top of the Edit Media screen, you will find a larger thumbnail of the media file, the file name, type, and the date when it was uploaded.
Click on the Edit Image button (button shown below), and it will lead you to an image editor. This section is further explained when you click on the Next link below.
The fields you can edit are the Title, Caption and Description.
Once you are happy with your changes, click on the Update Media button.