Window.old is here for Downgrade or rollback to previous Windows if you don't prefer the Newest Windows 10, so Microsoft time-scheduled this for one month, after when the Rollback option won't work, thus you don't have to worry about the size of the Windows.old folder, it will be back after 30 days, but if you want to delete Windows.old manually, we have many ways to do .
Where's Windows.old Folder locates? Press Win+E to open Windows Explorer, click This PC to list all the partitions, (Windows 7 will directly open partition list with this shortcut, why would Windows 10 change this?); Double click System Drive, commonly it's C Drive, Windows.old just lays there with Windows Folder.
After finding this folder, you may say the error: Access denied if you delete this folder by simply press Delete bottom, or right-click to delete. In fact, Windows had programed this before to prevent mis-delete, even when you run as administrator.
Access Denied error will also show up in command prompt when you type delete command to remove this folder if you are not running command prompt as administrator.
There are 3 ways to delete Windows.old in Windows 10: Disk Cleanup and Command Prompt