If you have saved your changes then you cannot undo them.
If you haven't saved your changes, then, you can close your Sitebuilder (without saving) and your site will revert to the old one.
If you have made site-wide changes and saved them, unfortunately, there is no "undo" button.
If you haven't updated (republished) your site, then, you can use your live site as a guide to recreate your site and reverse your changes.