This is how to use replace the default BBEdit icon with the BB-8 edition:

1. Click the link in the title.
2. Download the file which contains the icons.
3. Unzip the archive.
4. Open the BB8dit@1x.png file with Preview.app.
5. Press ⌘A to Select All.
6. Press ⌘C to copy the image to the clipboard.
7. Open a Finder window and go to /Applications/ or wherever you have BBEdit installed.
8. Highlight BBEdit and press ⌘i to bring up the information window.
9. Click on the BBEdit icon in the top left corner of the window — it should highlight with a blue outline.
10. Press ⌘V.
11. Close the window.

If you ever want to revert back to the original icon just open up the info window in step 8, select the icon and press Delete on your keyboard.

Thanks @jimmyhartington!