If you’re just a friendly neighborhood Nexus owner with a slight curiosity about what Android N might offer you, the Android Beta Program was made for you — you could just download the latest OTA package made available at Google’s discretion and enjoy.
But if you’re a developer that needs to hop between different preview versions and you hate flashing the full image on and reconfiguring your settings and re-uploading your apps and re-dreading your life … well, there’s a way out of that, too.
Google has made its over-the-air update packages available for flashing on its Developer site. All you need to do is ADB sideload them to your applicable Nexus. It won’t wipe your device.