Along with the release of Android 6.0 Marshmallow factory images, Google has also started to roll out OTA (over-the-air) updates to Nexus devices. Currently Android 6.0 OTA updates (build version MRA58K) are available for Nexus 9 W-Fi/LTE and Nexus 7 2013 LTE variants. However, your device should be running on the latest Android 5.1.1 Lollipop (LMY48T) to get the OTA update.
It will take some times to reach the Android 6.0 Marshmallow OTA (over the air) update notification on your Nexus devices. Check it by going to Settings -> About Phone -> Software Updates. However, if you are excited to try the Android 6.0 Marshmallow on your device right now, you can install the OTA update (build version MRA58K) manually.
In this tutorial, we let you know to install Android 6.0 Marshmallow OTA update manually on Nexus devices.
However, your device should be running on stock recovery (not CWM/TWRP) and updated to Android 5.1.1 Lollipop (LMY48T).
Important: Do not forget to back up the data of your device, before performing a manual installation.
1. Download Android 6.0 Marshmallow OTA (MRA58K) update zip file for your Nexus devices: Nexus 7 2013 LTE [Download], Nexus 9 LTE [Download], Nexus 9 Wi-Fi [Download]. OTA update for other devices (Nexus 5/Nexus 6) will be available later.
2. Transfer the downloaded firmware zip file to your device’s memory.
3. Now, boot your phone into recovery mode. To do so, switch off your phone, then press and hold the Volume Up + Volume Down + Power buttons simultaneously until you see the fastboot screen.
4. From fastboot screen, select recovery using power button and press the Volume Up key to boot into recovery.
5. From recovery, go to “apply update from external storage” using volume key and select it using power button.
6. Locate the OTA ZIP update file (you have saved on internal storage) and select it using power button. This will start flashing the Android 6.0 Marshmallow OTA update on your device.
Once the update process is completed, reboot your phone. It will take some times to complete the process. (Source – XDA)