The major new feature is the new app Health that can be synced with your smartphone Health Kit. The Pebble Time smart watch is becoming more and more interesting with each new update and this one is a major one.
You need to update your Pebble Time companion app on your smartphone. Then you will be able to upgrade your watch in just a few minutes.
At the same time, the original Pebble will also benefit a major revision as it is moving the the new Pebble Time interface. I just finished installing it on my OP. Even if the display is monochrome, the new interface is looking great, fast and smooth.