Google has always released the developer preview images of upcoming versions of Android in factory images format that is usually meant for developers and advanced users. This time around, with the release of Android N developer preview, the company has launched an Android Beta Program through which it will send registered users an OTA update to the developer preview build of Android N.
Interested users will first have to first enroll into the Android Beta Program, which hardly takes a minute or two. As soon as they register themselves, an OTA update will be sent to your device.
Google will also push the future Android N developer preview updates straight to your device through an OTA, so you never have to get down and first with flashing the factory image yourself. Once the final version of Android N is released, Google will send a final OTA update to get your Nexus device up to date with the public release of Android N.
You can un-enroll your device anytime from the Android Beta program, but doing so will also require you to format your device once, which means you will lose all your installed apps and their associated app data.
You can sign-up for the Android Beta program here. The list of eligible devices for the program include the Nexus 6P, 5X, 6, Nexus 9, and the Pixel C.