Since changes are saved as you go, there really isn't a way right now to undo a change in the editor once you've made it. There's a post on our Vote on Features board that I'd recommend adding kudos to if you'd like us to implement a feature like that, though.
Note that this only applies to the Weebly drag and drop editor. The Square Online editor used for Square Online sites and some Weebly sites does have an undo option. You'll find the arrows for that at the top of the website editor.
