Google has unveiled the developer preview of its next Android version known as Android L developer preview. The Android L has introduced many changes specially in multitasking. Developer preview is currently available only on Nexus 5 and Nexus 7 (2013). You can download and install the Android L preview in your nexus 5 or nexus 7 by following the instructions below.
First of all you must know that as it is a developers preview, it is not stable. You should not install it on your primary device which you use daily. And you should know how to use command line to troubleshoot if something happens to your device. Google also gives you a strong warning on the downloading page that this version is not stable and you may experience some problems.
First of all backup your important data and then download the system image for your respective device from Google Developers Site. A paid developer account is not necessary to download the system image.
When you download the system image then go to this link for instructions about how to load it. Once it is installed in your device, it will take bout 10 minutes to boot your device for the first time. If you get in to any problem, this Android Beat Tutorial will help you.