WARNING: Deleting the Windows.old folder will make it impossible for you to roll back to your previous Operating System or Build.
Open File Explorer C:\ locate Windows.old, right click and choose properties
Choose the Advanced button on the property dialog page
Change Owner to your user account, which must be a member of the local administrators group, and Change permissions to include your user account explicitly
When changing owner as well as when adding your account to permissions, step through the Select a principal routine and simply type your user name and click the check names button]
Be sure to check the "Full control" check box when adding your account to the permissions list.
After clicking the "Disable Inheritance" button you'll get the following prompt, simply choose, "Convert"
Double check and make sure you've checked the "Replace owner on subcontainers and objects" under Change owner, as well as the "Replace all child object permissions entries with inheritable permission entries from this object" so as to cascade your changes to all sub-folders and files.
That should do it and assuming you've clicked all the appropriate "OK" buttons along the way, and answered in the affirmative when prompted if you wish to continue you should be able to delete the Windows.old folder and end up with this
IF, I've forgotten any steps or something isn't clear.....
Let me know and I'll edit the thing to fix it.