How can i update my apple watch os 4.3.1

Update your Apple Watch from your iPhone



If a new update is available, your Apple Watch notifies you. When the notification comes, your Apple Watch and iPhone charging overnight so the update can complete.

You can also check for updates manually by the following process. It is important to keep your Apple Watch on its charger until the update completes.

On your iPhone, open the Watch app, then tap the My Watch tab.

Tap General > Software Update.

If asked for your iPhone passcode or Apple Watch passcode, enter it and wait for the progress wheel to appear on your Apple watch. It could take from several minutes to an hour for the update to complete.

Note: Don't restart your iPhone or Apple Watch, and don't quit the Watch app. When the update completes, your Apple Watch restarts on its own.

I think, above learning, topics help you but if more information see https://support.apple.com/en-us/HT204641

Start here;

Update your Apple Watch – Apple Support