I am setting up Google Apps for Business for a client. They had someone set it up previously for and it has sat there dormant not being used. When they validated the primary domain they did so via the 'upload HTML file to website' approach as described - here.
Anyhow this seems to rely on the HTML file being left in the domain root permanently which I don't like the sound of in case someone accidentally does something with it. I would much prefer to use the validate domain via editing the DNS record which seems more solid. But can't seem to find how to change it or revalidate it in the Google Apps for Business interface.