Android 6.0 Marshmallow
Now you need to start it in fastboot mode — to do this on the Nexus 6 press and hold down the Volume Down and Power buttons on the handset. How To Install Android 6.
This application is capable test your device and determine if it is capable to run the most recent firmware. Any of the selections will do just fine as you will have at least the Google Play Store from both of them, to install remaining Google apps if need be. For more of an Android 6.
Android 6.0 Marshmallow - Caution Please do not update firmware unless you know what you are doing, you really want it or it is absolutely necessary.