You contact me @elrumo on Twitter or by email.
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.
Download the template for Sketch, Figma, Pixelmator, Photoshop, Pixelmator or Illustrator from the Resources page.
Here's what you need to know know:
Credits can be seen in the website