Before you get your hopes high, the mentioned handset is the google play edition of the galaxy s4 which is in control of google to update and release patches.
The galaxy s4 google play edition is a handset that is similar to the Samsung galaxy s4 but has a nexus user experience with pure android os devoid of bloatware or the touch wiz. Google had a program which has be discontinued long back where it releases a google play edition of the popular flagship handsets such as the HTC one m7 and LG G2. The official update is now live and if you have a google play edition of galaxy s4, you can simply update by going to settings>about phone>system software updates. There will be a notification and you can update over the air.
It is curious how google is updating a phone that is beyond its 18 month update cycle. I guess the stagefright attack has something to do with it. The version that the phone will update to the lollipop 5.1 and not the 5.1.1 version. Still it is a result in positive direction considering the phone ran the old KitKat software before.
People who have a normal galaxy s4 need not be disappointed as a custom rom which updates the phone to 5.1.1 has be made available but you will have to root your phone to update.
If you want to install the custom rom, follow the link.