The best way to delete the Windows.old folder is not to delete it in Windows Explorer, but to use Disk Cleanup.
- Open the Start menu
- Search Disk Cleanup
- If you have multiple drives select the one that you have Windows 10 installed on
- Click on “Clean up system files”
- If you get the User Account Control prompt click “OK”
- If asked again to select a disk, select the drive that Windows 10 is installed on
- Check the box next to Previous Windows Installation(s)
- Click OK
- You will now be asked to confirm the process to delete the files