At the Google event on September 29th, 2015, it was announced that within a week’s time Google will be bringing Android 5.0 to the Nexus devices. The Nexus devices that were supported for the update to Marshmallow were: nexus 5, 6, 7 and 9. And keeping in line with their promise, Google has made the factory images of the latest update available for the Nexus phones and tablets on its website, where you can download them and flash them on your device manually. The OTA for the update will start soon as well, so in case you are not interested in going ahead with the flashing procedure you can wait.
The Nexus 5x and Nexus 6P were the first smartphones that were announced with Android M out of the box and since then there has been a constant wait for those who have Android devices about when will their devices get the update. The Nexus phones are always the first ones to get the update, and the story is the same this year. You simply need to visit the images page here and follow the instructions written on how to flash the images to the device.
This will take your device to the version 6.0.0 and the build number is MRA58K.