Make sure your Apple Watch has at least 50% battery
Keep your Apple Watch on its charger
Keep your iPhone nearby
Connect your iPhone to Wi-Fi
Open the Watch app on your iPhone
Tap General
Tap Software Update
Download and install the update if one is available
Enter your iPhone passcode if prompted
Wait for the update to complete
Keep both devices close together until the Apple Watch restarts
Check the Watch app again to confirm the update is finished