Install Android Lollipop Developer Preview
NOTE: If you've already installed a preview of Android, skip to step 2.
Step 1. Download the minimal 2MB installer from this XDA thread this XDA thread which includes the Android ADB and fastboot tools.You need these to install the Android L image, and this is the quickest and easiest way to install the tools without downloading the entire Android SDK, which is huge and contains stuff you don’t need to get Android L.
Follow the instructions (don’t change the installation path when asked where to install it) and you should end up with a command prompt window
Step 2. Download the appropriate file for your device:
Extract the files to the same folder as the Minimal ADB and Fastboot
Once extracted you should have a list of files as per the image below. (Note you do NOT need to unzip the zip file.)
Step 3. On your phone or tablet, head to Settings, and scroll down to About phone at the bottom. Scroll down to Build number and tap it 7 times. This unhides the developer options, which aren’t displayed in Android 4.2 and later.
In the Developer options menu, enable USB debugging. You'll need to do this even if you are running the preview version of Android L. And if you are, now skip to step 6.
Step 4. Download the Google USB driver from here here and extract the zip file somewhere.
Step 5. Now, Power off your device, and leave it connected to your computer’s USB port. Now start it using the appropriate key combination to put it in “fastboot” mode.
For the Nexus 5, that’s by holding down the volume up, volume down and power buttons simultaneously. Press power and volume down on the Nexus 7. Alternatively, you can leave your device on and type adb reboot-bootloader to restart in fastboot mode.
Step 6. If the LOCK STATE says locked (which it should) then type ‘fastboot oem unlock’ at the command prompt, without the quotes, and press Enter. This WILL erase your device, so make sure you’ve backed everything up first
Step 7. With the bootloader unlocked, you can now install the Android Lollipop firmware. Simply type
flash-all and the process will begin.