Do you wonder, how to delete the copy of your old system C:\Windows.old? But be aware of loosing data. Deleting this folder prevents you from roll back to a previous version of Windows. And you must be an administrator, otherwise you wont be able to delete it.
Just type in Windows search Cleanup Disk (in Czech Vyčištění disku)
Select the app Disk Cleanup
Select Cleanup System Files – be patient it might took some time.
Now you should see Previous Windows Installation(s) item in checkbox menu.
If you are really sure, select OK and its done.