Help to update app to Universal app

Question

I need again small help, I had my Windows 8 Store apps on the Store, I want to update those to Windows 8.1 and Make those all Universal app so that those all apps will work on Windows Tablet, Desktop as well on Windows Phone, So Can you give me help except the link of MSDN, cause I already checked with the MSDN Link.

Regards,
Aditya Patil