Nexus 5 Manually Install Android 6.0

Refer to https://www.androidexplained.com/boot-nexus-5-fastboot-mode/
Refer to https://developers.google.com/android/images#hammerhead

Download and Install the Google USB Drivers
Reboot Your Computer
Enable Developer Mode on the Nexus 5
Go Back to the Main Settings Page
Scroll Down and Tap on the ‘Developer Options’ Menu
Enable the ‘USB Debugging’ Feature
If Available, Enable the ‘OEM Unlock’ Option Too
Download and Install ADB & Fastboot Tools
Launch the ADB & Fasboot Tools Shortcut
Type the Following Command…
adb devices
…and Press Enter
Connect the Nexus 5 to the PC
Tap the ‘OK’ Option to Allow USB Debugging if Prompted
Boot the Nexus 5 into Fastboot Mode
Reconnect the Nexus 5 to the PC and Type the Following Command…

fastboot oem unlock

On the Nexus 5, Confirm That You Want to Unlock the Bootloader

fastboot devices***** device

When the Nexus 5 Bootloader is Unlocked, Reboot Back into Fastboot Mode
Download the Android 6.0 Marshmallow Factory Images

sha256sum hammerhead-m4b30z-factory-625c027b.zip625c027b21afe6de7c3d0de66e3a42000269dd00c2ef9a5347007734537f3ea2

Extract the Contents of the .zip File
Copy the Contents(all 6 files) of the .tgz File into the Minimal ADB & Fastboot Installation Directory
For Me, that was “hammerhead-m4b30z/flash-all.sh”
Type the Following Command…

./flash-all.sh

Wait for the Installation to Complete

target reported max download size of 1073741824 bytessending 'bootloader' (3124 KB)...OKAY [  0.307s]writing 'bootloader'...OKAY [  0.551s]finished. total time: 0.859srebooting into bootloader...OKAY [  0.003s]finished. total time: 0.053starget reported max download size of 1073741824 bytessending 'radio' (45489 KB)...OKAY [  1.630s]writing 'radio'...OKAY [  3.129s]finished. total time: 4.759srebooting into bootloader...OKAY [  0.000s]finished. total time: 0.050starget reported max download size of 1073741824 bytesarchive does not contain 'boot.sig'archive does not contain 'recovery.sig'archive does not contain 'system.sig'archive does not contain 'vendor.img'Creating filesystem with parameters:    Size: 13725835264    Block size: 4096    Blocks per group: 32768    Inodes per group: 8144    Inode size: 256    Journal blocks: 32768    Label:    Blocks: 3351034    Block groups: 103    Reserved block group size: 823Created filesystem with 11/838832 inodes and 93654/3351034 blocksCreating filesystem with parameters:    Size: 734003200    Block size: 4096    Blocks per group: 32768    Inodes per group: 7472    Inode size: 256    Journal blocks: 2800    Label:    Blocks: 179200    Block groups: 6    Reserved block group size: 47Created filesystem with 11/44832 inodes and 5813/179200 blocks--------------------------------------------Bootloader Version...: HHZ20hBaseband Version.....: M8974A-2.0.50.2.30Serial Number........: 066421e70ac569b6--------------------------------------------checking product...OKAY [  0.100s]checking version-bootloader...OKAY [  0.100s]checking version-baseband...OKAY [  0.100s]sending 'boot' (9156 KB)...OKAY [  0.500s]writing 'boot'...OKAY [  0.782s]sending 'recovery' (10014 KB)...OKAY [  0.568s]writing 'recovery'...OKAY [  0.824s]erasing 'system'...OKAY [  1.347s]sending 'system' (1020657 KB)...OKAY [ 32.205s]writing 'system'...OKAY [ 83.703s]erasing 'userdata'...OKAY [ 14.940s]sending 'userdata' (137318 KB)...OKAY [  4.541s]writing 'userdata'...OKAY [  9.158s]erasing 'cache'...OKAY [  0.665s]sending 'cache' (13348 KB)...OKAY [  0.640s]writing 'cache'...OKAY [  1.073s]rebooting...finished. total time: 151.822s

Then Press Any Key to Exit the Command Prompt
Wait for the Nexus 5 to Reboot on its Own
Be Patient Because the First Boot Can Take a While(it take 15 min in my phone to boot up.)

enjoy it!