After successful upgraded from Windows 8 to Windows 10, everything is working fine, and found the C drive free space drastically reduced quite a lot, almost 20 plus GB, googled and followed the online instruction to delete the previous windows installation, the link is as ​​http://www.tenforums.com/tutorials/2066-windows-old-folder-delete-windows-10-a.html​​

The previous windows installation was stored in the folder "Windows.old"

The size of this folder is around 16.4 GB

Run  the application "cleanmgr", and "Clean up system files", you will get the select to delete the previous Windows installations, and warning you about you cannot go back to old system if you delete the files. Click OK, files/folder will be deleted and the space will be freed for you.