Install the favicon just like the WordPress tutorial is, but because you are using (or about to use) the child theme, copy the header.php file from the parent theme (Celestial Lite) and paste it into the child theme, where you see the style.css file.
If you activate the child theme, WordPress will see it as a new theme being activated, so widgets will more likely have to be reassigned again, and any theme options you did such as colours will also have to be redone.
However, if you plan to make future modifications to the theme, you would then do it with the child theme instead so that any updates for the parent theme, you won’t lose your changes.
With a backed up theme, should anything go wrong, you can always revert back by uploading the theme and overwrite the files of the newer one.
If you decide to keep the parent theme and make modifications there instead of a child theme, make sure then whenever there is an update to the theme, that you backup your theme first, read the Changelog.txt in the theme, and then you can decide if you want to update or update only the files that were changed from the update.