Google has announced the Android 5.1 Lollipop update, the first major update to Android Lollipop and also released factory images of Android 5.1 for Nexus 5, Nexus 10 and Nexus 7 (WiFi) devices.
Support for multiple SIM cards.
Device Protection – lost or stolen device will remain locked until you sign in with your Google account even with factory settings.
high definition (HD) voice calling.
improves stability and performance.
ability to join Wi-Fi networks from Quick Settings.
control of your paired Bluetooth devices from Quick Settings.
Download the Factory Images for Nexus Devices