Does Visual Studio 2015 include .edmx support?
I have a project that includes an .edmx data model and double-clicking it in VS2015 just opens the XML editor.
The
You have to select the Microsoft SQL Server Data Tools in the VS2015 setup to get this editor back.
NOTE: You can modify the current installation via Program And Features as magicandre1981 has already suggested.
see how this is done here