Factory images and OTAs can be found at the Android Developers site. The following links below are factory images for build NPD56N. You can learn how to install Android N Developer Preview factory images using our simple guide.
Instead of flashing and wiping your devices, Google also provides download links to the same OTAs that are rolling out as we speak. You can use these if your device already has Android Developer Preview 4 for a faster update. Simply follow our guide for installing Marshmallow OTAs if you need to learn how to do this.