This one was out late on 3/21/16 but you may not have noticed. It did not prompt or anything on my phone or watch. But if you go into the Watch app on your iPhone, and select General \ Software Update you will find the update.
Do not forget that once you start the update you will need to select proceed on the watch. I missed that for a bit. It does take a while – maybe 45 minutes? There is near the top of the Software Update screen an indication of time remaining – see screenshot below. Not sure how accurate – I mean after it counts down, it is not done but rather at the prepare stage for a while, followed by verifying, and finally installing – which takes a while. This seems like a more complex update then others I have done. Longer anyway, and the fact I had to acknowledge on the watch.
I have only one Apple Watch, and it updated fine.