https://www.pcgamer.com/what-to-do-with-windowsold/

If you've updated Windows 10 recently (or upgraded to Windows 10), then you probably have a folder on your system called Windows.old. Maybe you found it while searching for big files clogging up your drive, or this is news to you. Why is it there? What do you do with Windows.old?

Although for most people Windows.old is redundant and can be left alone, deleting it could be a good way to free up a ton of space, especially on a small SSD. Read on to find out how to get rid of Windows.old, and whether or not you should.