Upgrade Firmware

1. Backup all your data on your nexus 5 as flashing this official firmware might result in data loss.
2. Charge your handset to at least 80% so that flashing goes well without any issues due to low battery.
3. You must have installed adb or minimal fastboot on your computer.
4. Download the developer preview for hammerhead on your computer.
5. Download and install win zip for any standard extractor tool on your computer.
6. This method can also be used on other nexus devices like nexus 6 shamu, nexus 9 volantis.

Install Android marshmallow on Nexus 5 hammerhead:

• System ui tuner: This is new feature that let you tweak ui a bit but has been hidden just like developer option.If you want to add it to setting app,swipe down the notification panel and on top you will notice a gear icon.Press and hold it and you will notice that it starts spinning.Just leave it once you notice the vigorous spin and you will see a toast message "system ui tuner" added to settings.
• There are ton of brand new wallpapers.
• Google launcher now has vertical scrolling instead of horizontal.
• Multi window: Google finally introduced multi window concept in tablet version of os.You can run two apps in a single window.Activating multi window is also too easy.When you open recents you will notice there is another option beside the closing button on cards.Press on it and activate multi window.But this multi window is not configurable which means adjust width of apps that are running.
• Phone short cut on lock screen is replaced with google now.
• Search bar in settings menu.
• New volume panel: You can now adjust volume of media, call and notification from single window .This feature is absent in previous versions of android though present in most of the oem provided firmwares.
• Auto rotation on home screen is also added which you can find in launcher settings.
• Now on tap: If you use google now frequently this feature might help you a lot.Usually when you press and hold on home button it will take you to google now.If you enable this feature when you try to access google now on any app it load some related suggestions along with some google now card.
• Screen shot notification will also have delete button along with share option.
• Memory option: This is available in settings .It gives you memory usage on your device from last "3hrs or 6 hrs or 1 day".
• App permission manger is the major thing in android m.This lets you to block certain permission so that the app cannot access them.like if you don't want facebook to access your bluetooth you can now do it with out rooting your device,

• Unlocking bootloader is the first step that you have to go through before installing android m on your nexus 5.But unlocking will erase all the data on your device so have a backup first before proceeding further.
• Download and install lg device drivers on your computer.
• Come back to phone and press and hold power button and choose power off.
• Hold volume up, down button and power button simultaneously and you will notice that your device goes into fastboot mode.At this stage connect to computer using usb cable.
• Once you are in fastboot mode connect your nexus 5 device to computer through usb cable and if you notice any driver installation process let it complete first.
• Download  fastboot on our laptop or Pc and install it.You can install it any location but choose location as c: which is default location.
• open the fastboot folder which is located in "c:\programming files".Right click on empty space by holding shift and you see an option "open cmd here".
• If you don't find this option then press windows and R buttons at same time ,Run pops up and type cmd to open command prompt.
• Now go to fastboot folder and copy the location of it which you can see in address bar of folder.
• In command prompt type the command "Cd location of fastboot folder" and press enter.You will notice that fastboot appears in command line.To paste location right click on empty space in cmd and several pops up.Select paste.
• One you press enter you have to notice minimal adb and fastboot in command line instead of c or some thing.
• Once you are in fastboot window type "fastboot devices" and press enter.If your device is detected you will get a serial number in list of  devices attached.If not uninstall drivers and install them again.
• Now type the command "fastboot oem unlock" and press enter and you will get a prompt on your nexus 5 if you want to unlock your handset.Click on yes and this will erase all your data or info on device.It reboots again after unlocking.
• Download android m developer file on your computer and extract it using 7zip or winzip.
• The extracted file will have flash-all.bat file along with all other imgs like bootloader ,system ,recovery,boot and all.
• If you don't find all images and if you see another folder that contain all other images cut then from that folder and paste it in the parent folder.
• Now double click on flash-all.bat file and it flashes all the files for you.Once the process is completed you will have android m installed on your nexus 5.
•  If flash-all.bat file failed to installed then you have to flash the image files manually.
• Copy all the image files from the extracted folder and place them in fastboot folder.Put your nexus 5 in fastboot mode again and follow the process.
• Now type the command "fastboot flash bootloader bootloader.img" (here this green underlined word indicates the name of the bootloader file.It may not be exact word.You have to type the name which you can see in the fastboot folder)
• Once the bootloader is flashed reboot it bootloader again by typing "fastboot reboot bootloader".
• Now type the command "fastboot flash radio radio-filename.img" and hit enter.Once it is flashed reboot to bootloader again.
• Now type the command "fastboot flash system system.img" and dont reboot to bootloader this time.
• Now type "fastboot flash userdata userdata.img" once flashed just continue to next command.
• Now type the command "fastboot flash boot boot.img".
• Now type the command "fastboot flash recovery recovery.img".
• Now erase cache using the command "fastboot erase cache" and flash cache img too using the command "fastboot flash cache cache.img".
• Now type the last command which reboot your nexus 5.Type "fastboot reboot" and hit enter.You will notice that the your device reboots.