To deploy the latest Apple Watch software update on your device, tap My Watch → General → Software Update in the companion Watch app on your paired iPhone, then follow the onscreen instructions to download the update over the air.
Your iPhone must have iOS 11.3 for the watchOS 4.3 update to show up.
Read the watchOS 4.3 release notes for detailed information on feature improvements.
Nightstand mode, for example, is no longer solely limited to landscape mode—now it works both vertically and horizontally, which will come in handy when charging your watch using Apple’s upcoming multi-device Qi wireless charger.
The AirPlay menu accessible via Control Center now lets you send audio to the device’s built-in speaker in case if you’d like to listen to music through the tiny speaker.
The iPhone music browsing and control has been reinstated in watchOS 4.3 and there’s a new nicely done animation when you connect your watch to a charger.
For information on the security content of watchOS 4.2, read Apple’s support document.