In the Program and Features, first select the VS 2015 and then click on “Change”. Go to Apple developer accounts window in Visual Studio.
Once you click on Program and features you can see the below screen. (If Using Windows 7 images may vary because I am using Windows 10)
I will explain to you what the Universal App is in my next article/blog, first, let's see how to Add/Install the Xamarin in Installed Visual Studio 2015. I will suggest you use Windows 10 because Xamarin allows you to create the Universal App in Windows 10 only. In my case, I am using the Windows 10 Operating System and Visual Studio 2015 Enterprise edition. Please close all the running instances of Visual Studio 2015. If you have already installed the Visual Studio 2015 and want to Install Xamarin then please follow these steps.