Elias, Creator of http://macOSicons.com. You can contact her through twitter @elrumo.
Beautiful, ready-to-use Icons for MacOS.
Download one of our free icon templates from the resources page.
For a more visual guide checkout our Instructions page.
You just need to download the icon you want, find the app to update on Finder, select it and "File > Get Info" or "⌘-I" and drag and drop the downloaded .icns file to the new window.