You can edit Classes/Unity/DeviceSettings.mm and Classes/iPhone_Common.h, I guess. Am sure a future version of Unity will support iPhone6 and iPhone6Plus identification. You should be able to build onto these new devices just as soon as you have one.
At the moment iPhoneGeneration doesn’t have definitions for the new iPhone 6/6+ but Xcode will report back correctly the new device IDs. As a temporary hack you can use this: