The OS was originally scheduled to debut alongside iOS 9 on September 16, but shortly before it was ready to make it’s way to wrists everywhere, Apple noticed a big bug which needed to be taken care of, which ultimately delayed the launch of the 1st major update for Apple Watch.
If you’d like to install the new update for your Watch, open the Apple Watch app on your iPhone and tap through My Watch > General > Software Update. Make sure your watch is in Bluetooth range of your iPhone and connected to its charger. Then install it. The installation will take a while, so be patient.
You can view Apple’s official page for watchOS 2 here to learn about all the new features.