Google has long made Nexus and Pixel C factory images available for people who just can’t wait to install new Android versions (or want an alternative restore option). However, they’ve always come with a big gotcha: since it’s a complete system image, you have to wipe your phone clean. You won’t have to start from scratch after today, though. Google has posted over-the-air update images for all its currently supported Nexus and Pixel C devices, giving you the satisfaction of an immediate upgrade (no weeks-long waits here) without nuking all your data.
Source: Google Developers