While still in the General tab, select the arrow to the immediate right of the App Icons Source drop-down menu:
Afterwards, you will be taken to a screen that will let you drag and drop image assets to their corresponding logos. Xcode will provide warnings if you end up adding an incorrectly sized icon to the wrong drop zone:
The text underneath each drop zone's section shows the target resolution for each icon.
Once you have your app icons set up, it's time to create an archive of your application for submission to ...