new simplified ribbon won't toggle back to the classic ribbon on "excel for the web"

I've been sharing my Excel doc with my daughter on Dropbox for over a year. She opens it with "Excel for the web." Suddenly, when she opens the doc, she can no longer see the classic ribbons which enable her to utilize tools. Instead a single headline appears at the top and the space where the ribbons were initially are now invisible. In that single headline there is what appears to be a toggle switch with the words "simplified ribbon" next to it. However, she is not able to toggle it or get it to do anything at all no matter how much she clicks on or around it. I'd appreciate any help with this. Thanks. ~Sherry