Flash Upgrade Nexus Android Devices

 Android phone firmware

1. Install Windows portable Minimal ADB and Fastboot, Unzip and run MAF32
2. Enable Android Debug Bridge abd debugging on device,  Settings > About phone and tap Build number seven times. Return to the previous screen to find Developer options at the bottom.
3. Booting into fastboot mode fastboot oem unlock
4. download Official Android Images and unzip it, copy ADB files to it and run flash-all

XDA-developers forum
Nexus 6 shamu google, Lineage OS
Nexus 9 volantis
Nexus 5 hammerhead, AOKP (popular), AICP, AOSP, LineageOS 
Nexus 4 occam
Nexus 7 Mobile nakasig
Le Max 2

LineageOS (CyanogenMod)

Improve battery life with Xposed, Amplify and Greenify – Android customization
Extreme Battery Life Thread (Greenify+Amplify+Power Nap)