I bet it's dead simple. So I'm going to 😱 before I even read the replies. I have changed icons before by pressing Apple + i on an App and the same on another icon then copying the icon's preview onto the other App.
I thought that, but I got followed the same steps and the .icns file doesn't show a proper icon in the upper left. Just a generic icon. 😱
Found my solutions for anyone interested:
1) Open the .png file in Preview
2) Press command-A to select the entire image
3) Press command-C to copy the selection
4) Control-Click on the file/folder you would like to change
5) Click the icon (so there is a frame around it)
6) Press command-V to paste the image into that