Here is my tutorial how to upgrade Motorola Nexus 6 (shamu) to latest Android version manually. It is universal walkthrough for both rooted 6.0.* (Marshmallow) and 7.0.0 (Nougat).
Complete walkthrough how to root Motorola Nexus 6 (shamu) running Android 6.0.1 (Marshmallow).
To mess around with Android-powered devices you will need Android SDK tools: Android Debug Bridge (or ADB) and Fastboot. Here's how to install them on Linux Mint.