At first I tried to run cleanmgr as a regular user, and it didn't list Windows installation files and Windows previous version files as options for clean-up.
The system will go through the cleanup process and you will regain that disk space.After that reverting back means reinstallation of your former version of Windows.So with all that known – once you have decided that your install of Windows 10 is good to go and you are satisfied then you can carry out this clean up task. You should see the Disk Cleanup program entry at the top of the search results (2). When the program window opens it defaults to the hard drive where your Windows installation is located. After the initial scan is done you must then select the option to (1) and then begin the scan again by clicking OK. In these two screenshots, thanks to @Local Joost & @jussipalo for the images, you can see an entry labelled and free up all of that disk space.Every time you upgrade your PC from an older version of Windows to Windows 10 without first formatting the hard drive, or when you install a new Windows Insider Preview test build, the install wizard will save a copy of the previous version inside of the "Windows.old" folder on your computer.This copy is a safeguard mechanism in case something goes wrong during the installation process.