Which Apple Watch are you using? Is it being charged to at least 50% and still on the charger?
The iPhone is being charged as well and near the watch?
Did you already open the WatchApp on your paired iPhone and check the update process in My Watch -> General -> Software Update?
You may also try these troubleshooting steps:
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- Make sure that your Apple Watch is connecting properly to its charger.
- Restart your Apple Watch.
- Restart your paired iPhone.
- Try again to update your Apple Watch.
If the update won't start:
- Open the Apple Watch app on your iPhone.
- Tap General, then tap Storage.
- Delete the update file.
- Try again to update your Apple Watch.
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copied from: Update your Apple Watch - Apple Support