I need to make changes to someone's robots.txt file, but their site is managed by Google (so no FTPing).
I have full access to the site via the browser (Site Actions / Manage Site, etc.) but how do I get to the robots.txt file to update it?
The robots.txt file is located at https://sites.google.com/robots.txt.
You don't have access to edit that file, it is automatically generated by Google.
The article you are referring to is generalized for how most website owners or admins can make changes.