This place allows you to edit the setting of the specific download item that has been already created. The screen that shows after pressing this button is the same as when pressing the Add button. The only difference is that here you are editing an already created item, whereas when pressing the Add allows you to create new items.
The only difference, when comparing this screen to the Add screen, is that there is one more field which shows the file that is downloaded and connected to the specific download item. This field is called Upload file.