Today, Apple released watchOS 4.0.1, which is meant to fix cellular issues that have been present on the Apple Watch Series 3, which was released last month. The device is the first Watch to support cellular networks, and has been plagued by connectivity issues.
For example, when leaving your home Wi-Fi, the device might attempt to connect to an unauthenticated public network, rather than working with cellular. The bug had to do with the handoff between Wi-Fi and LTE networks, and Apple had promised a software fix.
There doesn't seem to be any other fixes in the update, including security fixes. According to Apple, watchOS 4.0.1 contains the security improvements of watchOS 4, and it's only available for the new Apple Watch Series 3.
To download the update to your new Apple Watch, you can get it through the Watch app on your iPhone, which should also be running on iOS 11.