macOSicons
by Elías Back to resources 
1
Download the icon you want from macOSicons.com
Choose an icon and download the .icns file to a folder you can find quickly.
2
Find the app in Finder and open Get Info
Right-click the app in Finder and select Get Info to open the Info panel.
3
Drag the file onto the current icon
Drop the downloaded .icns on the small icon at the top-left of the Info window.