There are three ways to undo changes.
1. If you just made the change and the page builder is still active at the top left click “currently editing…”. Then click history and you can undo any change you’ve made there.
2. Another option is to click done then discard changes to undo all changes since the last time you published.
3. If you published the changes activate the page builder again then click “currently editing…” at the top left. Click revisions and choose a revision that has the content you want.