Google released Android 6.0.1 April security patches for Nexus devices, which fixes many security issues found in the OS. Factory images for the update are already available for download, however Google not yet started to rollout of the OTA (over-the-air) update for the patches, but it should reach on supported devices within a week.
April 2016 security update addresses 29 security bugs found in the OS and among them eight items were listed as critical. Most of the critical security issues were related to remote code execution vulnerabilities discovered in DHCPCD, mediaserver, media codec and libstagefright processes of the OS. You can read more about this at Nexus Security Bulletin here.
The OTA update for Android 6.0.1 April 2016 security patches will reach on your device within a week. However, if you cannot wait for the OTA, you can always flash the factory image by downloading it [HERE] from Google. Once updated, the Android security patch level will be April 2, 2016.
The below listed Nexus devices are currently eligible for April 2016 Android 6.0.1 security update.
Nexus 5 (MMB29X)
Nexus 5X (MHC19Q)
Nexus 6 (MMB29X)
Nexus 6P (MHC19Q)
Nexus 7 2013 WiFi (MOB30D)
Nexus 7 2013 LTE (MMB29X)
Nexus 9 WiFi (MOB30D)
Nexus 9 LTE (MMB29X)
Nexus 10 (LMY49J)
Nexus Player (MOB30D)
OTA (over-the-air) update is recommended for most users. However, if you prefer to flash it manually, you can get instructions for installing the update by reading this guide. (Source – Google)