I take that from your PSU, there goes two power cables to your MoBo, a 20/24-pin and a 6-pin. The 6-pin is used to power the CPU. Don't unplug it.
Just replace your GTX 560 Ti with GTX 950 and use old GPU 6-pin power cable with 6-pin to 8-pin adapter to connect the power cable to your new GTX 950.