I am new for this side. I wonder if it's easy to make the portability of a windows phone 8.1 application for a universal app?
What is the easiest way to do it?
thanks
You are going to have to do some porting of code, not all API's available for 8.1 are available in Universal Apps as yet.
The easiest way to achieve maximum code share is to add another project to your solution that targets Universal. When you have done this, I would then add a another project to your solution of type "Portable Class Library (PCL)" that can be shared between the 8.1 app and the universal app (as well as Xbox, Android/iOS with xamarin should future requirements need it). You can then move/refactor/port as much of you 8.1 logic into the PCL as possible. This approach should mean maximum reuse of your business/service/data code.