Newest version: Windows 10 including Win10 Anniversary Update (Win10 AU)
Older versions: Windows XP through Windows 8.1
Windows got a brand new version in July 2015 but previous versions are still widely used. So you need to match the lesson unit to your operating system.
Previously the lessons on Windows got tweaks added as new versions of Windows were released. But Windows 10 is intended to be automatically updated with new features along the way. No more whole versions in a box. So it does not make sense to add that version into the already-messy mix.
The lessons Working with Windows 10 will be updated as needed and the previous lessons will be frozen in place. They won't be shifted to the Archives section yet. Quite a number of businesses will likely use Windows 7 for years to come.