Hands on overview with the new Android L aka Android 5.0 the next version of Android on the Nexus 5, currently Andriod L is a Developer preview release (beta) and as of now Google has not announced any official public release date for Android L yet.
Android L Developer Preview Images for Nexus 5 & Nexus 7 http://developer.android.com/preview/setup-sdk.html
Do note that if you plan to flash the Android L image it will completely wipe any data on your Nexus device, also if you damage your device while doing so I am not responsible, so if you are not comfortable with flashing proceed with caution.
You can check this website for more instructions http://www.gottabemobile.com/2014/06/26/how-to-install-the-android-l-beta-system-images/
