How to properly remove the Windows.old folder on Windows 10

[Windows Central]

Did you upgrade to Windows 10 or install a new Insider test build? Then use this guide to get rid of the Windows.old folder that is only wasting gigabytes of space in your PC's hard drive.
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.

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