Terrain Tinker saves each version of your scene in case you want to revert back to a previous set of changes!
You can view the saved versions under File > Version History. Click Open to view any previous version in the editor.
While viewing a previous version the Version History window will continue to show the other previous versions of that scene as well as the original for easy navigation.
If you make changes to an opened version and press save, it will branch from the original scene with a version number post-fix so that your latest version is not overwritten.