How to download Visual Studio 2015 with key

Dear all please help me with this, I am using visual studio 2010. I also have visual studio 2013 but now I want to switch to 2015, so how do I download it so that I don't have to worry about registry key etc. One more point my windows application that I have developed from visual studio 2010, is this gonna be effected when I'll be building a setup file of it from 2015. Because last time I had the same error I developed the windows application from visual studio 2010 and made setup of it, but after I install visual studio 2013 I could not make the set up file of the application in visual studio 2013 which I developed in 2010, so unfortunately I switched back to 2010 again. Now I want to try 2015 and want to explore Xamarin including with all other windows and web features. Please help me with this. Thanks a ton in advance.