See below in DisplayManager.mm. look for UnityScreenScaleFactor:
Modify as below:
case deviceiPhone7: resMult = 0.75f; break;
As you can see above, the resMult is what you multiply the resolution accordingly. For example if you take a 0.5f multiplyer on an iPad Retina (2048x1536) it'll turn your resolution to 1/2 or 0.5 of it which is essentially 1024x768.