Undo Button

An undo button would be nice. I have searched through the community and seen numerous requests from many people, on how to undo an action. The only solution that I heard many people offer was to immediately click the disappearing "button" at the bottom of the page after discovering you want to undo something?

What if you want to preview the changes you have made, then decide to go back to the design you had already created? How do you undo the changes then?