So, you have been warned, but the good news is that you can uninstall the Android Lollipop preview and revert to factory settings. Just make sure you have backed up everything that you can’t afford to lose before you install Android Lollipop, since the process will wipe your device.
How to install Android LollipopNOTE: 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:
Once extracted you should have a list of files as per the image below. (Note you do NOT need to unzip the zip file. Leave this as it is).
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 5. With your Nexus connected via USB and powered on, go to the Control Panel and click Devices and Printers. You should see your device in the list. On our Windows 8 machine, it was listed as ‘MTP’ under Multimedia devices. Right-click on it, and choose properties. Click on the Hardware tab, then select the Nexus 5 in the list, and click Properties. Now update the driver, pointing Windows to the folder where you extracted the Google USB driver. Once done, it should report the device as Android Composite ADB interface. Plus, you should get a prompt on your device’s screen asking “Allow USB debugging?”. Tick Always allow from this computer and tap OK.
For the Nexus 5, that’s by holding down the volume up, volume down and power buttons simultaneously. You should see a screen like the one below. 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.
This WILL erase your device, so make sure you’ve backed everything up first.
and the process will begin. You will see alternating downloading… writing…. erasing… on your device’s screen.
Locking the bootloader won’t wipe any data, but if you unlock it again, it will erase the device of all user data and settings.
How to uninstall Android Lollipop and return to factory settingsReverting to the version of Android you had before installing Android L is pretty much the same process as you’ve just completed.
Step 10. First, download the correct version of Android KitKat for your device from Google's images page
You can choose which version of KitKat you want.
Step 11. Then, extract it as in Step 2 above. You’ll need to delete the Android L files from the c:\Program Files (x86)\Minimal ADB and Fastboot folder before copying the extracted files there. These are the files to remove (in case you forgot which were which).