How to update Apple Watch without Wi-Fi using cellular?

Sorry, but you will have to use a Wi-Fi connection, there is no way around that.

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1. Make sure that your iPhone is updated to the latest version of iOS.
2. Make sure that your Apple Watch is at least 50 percent charged.
3. Make sure that your iPhone is connected to Wi-Fi.

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If your Apple Watch has watchOS 6 or later, you can install updates without your iPhone:

1. Make sure that your watch is connected to Wi-Fi.
2. On your watch, open the Settings app.
3. Tap General > Software Update.
4. Tap Install if a software update is available, then follow the onscreen instructions.

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copied from: Update your Apple Watch - Apple Support

Ubhsuhk20 wrote:

how to update apple watch without wifi but with cellular?

You don't. WiFi is required.