Google just posted factory images for May’s Android security update. Devices receiving the update so far include the Pixel C (MXC89F), Nexus 6P (MTC19T), Nexus 5X (MCT19T), Nexus 6 (MMB30G and MOB30I), Nexus Player (MOB30G), Nexus 9 WiFi and LTE (MOB30G), Nexus 5 (MOB30H), and Nexus 7 (2013) WiFi and LTE (MOB30J).
You can grab each image at the link below. For instructions on how to flash a factory image, here you go.
Factory Image Link
UPDATE: Notes from the May security bulletin are now live, should you want to know exactly what was patched.