The latest Android Lollipop source code was released on 3rd November this year.
So the official builds will be released very soon but not yet available for most of the devices.
But xda senior member mo976 has ported the Lollipop CyanogenMod 12 for Google Nexus 5.
CyanogenMod is based on AOSP and CyanogenMod 12 will be the name for Android Lollipop version of CyanogenMod.
Installing custom ROM requires a custom recovery to be installed on your device, which may VOID your warranty and may delete all your data.
Before proceeding further you may consider backup your all data.
Please visit the Disclaimer page below to agree with the risks involved and proceed before flashing this ROM to your phone:
ROM Disclaimer link
Installing CyanogenMod ROM:
If you want to update your phone to the latest Android Lollipop based CyanogenMod 12 ROM then you have get the recovery flashable ROM zip file over here:
CyanogenMod 12(Lollipop) ROM download link
You can get the radio files over here:
Nexus 5 modem files download link
SuperSU rooting recovery flashable zip package by Chainfire:
SuperSU download link
Because this is an un-official build this is not hosted on Cyanogenmod official website and will get the updates only when the developer will release one.
The Nexus 5 has no built in EFS backup partitions. EFS contains things like your MEID or IMEI… your data profiles.. radio settings.
The EFS cannot be replaced… it is unique to your phone only… backing up your OWN EFS is highly recommended.
I will not be responsible for the consequences if you will not go for the EFS backup.
EFS backup link
Copy the ROM zip file to your SD card or internal memory(By using a USB cable or via adb push command).
After copying the ROM zip file to your SD card you have to reboot into recovery.
To go to the recovery type the following command (if you are on a PC):
$ adb reboot recovery
Wipe Data/Factory Reset
Format /system, clean Dalvik Cache
This step will delete all your data.
Consider backup your all data out of your phone.
Data Backup guide
Select the Install Zip from SDcard option.
Select the ROM zip file where you have kept in the SD card.
The flashing will take a while.
When it will be done you can reboot your system and enjoy the CyanogenMod 12 ROM on your device.
This ROM does not contain Google applications like Gmail, drive, maps etc.
You need to install them separately.
Below is the download link for GApps Lollipop.
Lollipop compatible Google Apps flashable zip download link
Download the GApps zip file from the above page as per the Android version that you are flashing, copy the file to your SD card and follow the above ROM installation guide to install this package too.
Do the same to install the modem and root package.
Stay tuned to get more updates