Hi Hamsajid,
Welcome to the HP Support Forums, I hope you enjoy your experience! To help you get the most out of the HP Forums I would like to direct your attention to the HP Forums Guide First Time Here? Learn How to Post and More.
I saw your post and will be happy to help. Are you looking to go back to Windows 8 from Windows 8.1 or do you have Windows 8 and you are looking to restore your original operating system? Here is a document on Performing an HP System Recovery for Windows 8.
Thank you,