Specify AppleScript bundle icon
Disclaimer: This may not work for an AppleScript bundle.

First I open the icon in preview.app.

I select the full picture and type Command-C

The way I typically change icons for files is that I select the file and type Command-I

In the info window that opens, I then select the current icon in the upper left.

Type Command-V and the icon (theoretically) will change to the picture in the clipboard