Updating windows from other drive
You can use it to restore your system to the old version of Windows if you don’t like the new version.If you’re looking for a specific file that wasn’t properly copied to your new Windows installation, you also could dig into the folder and find it.In Windows 10, click the Start button, search for “Disk cleanup,” and then launch the Disk Cleanup app.You can also right-click the C:\ drive in File Explorer, select Properties, and then click the “Disk Clean-up” button on the “General” tab. You’ll see an option for “Previous Windows installation(s)” appear in the list of things you can delete, and Disk Cleanup will tell you how much space those files are taking up.Click Start button and input "CMD" in the search box, and then click "cmd.exe" to launch the Windows Power Shell prompt.Once the Command prompt window pops up, input "diskpart". Input the command line "disk list" then press Enter button, after that all the drives of the host computer will be listed. This will clean the partition first, then a primary partition will be created.The new version of Windows just keeps it around in case you’d like to go back to that older version of Windows or in case you need to dig in and find a file.
Again, as the interface notes, this option is only available for a month after you upgrade.
Windows will automatically remove the folder to free up space after a month, so that’s how long you have to decide whether you want to stick with your new version of Windows.