Each slide has its own history. To see the history of a card click on the "History" icon on the top-left of the header.
You will see the list of changes you've done.
If you click on any registry, you will see the preview of the change on the selected slide. But it will not be saved, it is just a preview.
To restore your history, click on the "more" button on the right of the history item and select "Restore" option. Your selection will be saved.