After a Windows Upgrade, by default, a scheduled task is created and will run after four weeks to delete the Windows.Old directory. But if you wish, you can also remove the Windows.old folder manually earlier. If you find that you no longer have any use for it, you may safely go ahead and delete it as follows:
Delete Windows.old folder using Disk Cleanup
Open Disk Cleanup Tool and select Files from all users on this computer. Click the Disk Cleanup tab, and then look for the Previous Windows installations check box. You will need to click on Clean up system files button, in order to see this window.
This takes an age gathering info but its worth it. I also checked the Windows upgrade log files and the Temporary Windows installation files to remove as well.
install logs Windows.old directory