Few Notes I should point out before i tell you how it's done:
1. Undervolting isn't a point and click thing, it requires at least a few hours of stress testing as you have to stress test multiple voltage/clock points on a graph.
2. By itself, Pascal already does a great job at giving you the best performance per watt, so you aren't gaining that much more FPS and/or decreasing power consumption too much.
So there are two ways to do this, there is an easy way and a hard way.
The hard way is to manually tell the GPU what clocks go to which voltage points, you do this by installing an overclocking too, i recommend MSI Afterburner, then you go into the voltage graph (ctrl + k) and start playing around with each point and seeing what is the most stable at the lowest voltage. I'd recommend you watch the clocks at stock on a stress test first to see how your GPU behaves.
Doing this is very difficult as it is very time consuming, your basically underclocking 10 voltage points and each voltage point needs to be stress tested individually.
Or, you can go the easy way, simply go into an overclocking utility and lower your power limit to say 70%. Decrease or increase however much you want to gain the desired undervolt.