Deploying Android 6.0 in the current environment poses significant security risks:
Manual installation can potentially brick your device. Proceed with caution and at your own risk. android 6.0 marshmallow download
If you're comfortable with manual installation, you can download the Android 6.0 Marshmallow firmware for your device from various online sources. Deploying Android 6
For non-Google phones, you must use specific tools to download the firmware: android 6.0 marshmallow download