Every time you upgrade your version of Windows a copy of the previous installation will save on the hard drive. The operating system does this during installation in case things go wrong and rollback is necessary. Additionally, it’s a way to uninstall Windows 10, in case you upgraded by mistake or simply don’t like the new version.
Previously, we used to use the Disk Cleanup tool to delete the “Previous Windows installation” and “Temporary Windows Installation Files” to reclaim hard drive space. While the tool still works, the Windows 10 Anniversary Update brings the same functionality to the Settings app.
In the new Temporary files settings page not only let you delete temporary files, clean the Downloads folder, and empty the recycle bin, but it also includes an option to delete the “Previous version of Windows”.
In this video tutorial, you’ll learn the steps to delete temporary and unnecessary files from your computer. This also includes deleting the Windows.Old folder that contains the previous version of the operating system, and the $Windows.~BT hidden folder that contains the files Windows Update downloads to upgrade your computer.
As a reminder, note that deleting the Windows.old after folder upgrading to the Windows 10 Anniversary Update will prevent you from going back to a previous version. In addition, starting with the Windows 10 Anniversary Update, Microsoft has shorten the time the previous version stays on your computer after an upgrade from 30 to 10 days.
This is also good tip when Microsoft rolls out a new Windows Insider Preview build through the Fast or Slow ring. As you now every preview you install will create a copy of the old version that could be wasting valuable space on your hard drive.