This update to the software isn’t that hefty as it just weighs 81 MB and you are suggested to upgrade while you are on WiFi only. The update is available via Over the Air download.
It brings the firmware version as LMY47Z and offers stability improvements. The update is said to rolled out in phase wise manner and it might happen that it’s not available on your device right now. Don’t complain, wait for few days, it should definitely hit your device anytime. Until them be prepared to free up some space on your phone storage so that as soon as the OTA arrives you are ready to update. Also when downloading the update make sure that the battery life left on your device is atleast 50%. If you can't wait then download the factory image from here.
Android 5.1.1 Lollipop doesn’t add any feature to your current version as it’s meant to be a bug fix update.