#1. iFixit most likely doesn't have 6S screens in stock yet.
#2. The 6S screen is physically incompatible. Different connectors, different connector layout, not to mention the increased thickness of the screen due to 3D touch (That is why the 6S is 2mm thicker). Even if some how you could connect the display, I am sure the installed iOS of an iPhone 6 would not support the 6S screen.
#3. Not worth the trouble even if it were possible. Every year Apple touts increased glass strength and every year, people still crack their screens.