Can I change the voice on my apple watch?

Siri uses the same language and voice on your Apple Watch that's set up on your iPhone. On Apple Watch Series 3 or later, you can have Siri speak responses. On your Apple Watch, go to Settings > Siri.

On earlier Apple Watch models, Siri will display responses on your screen with text or images. Learn more about Siri on Apple Watch.

So you can only change your Siri voice if you also change it on your iPhone.

See this instructions on how to do that

Go to Settings > Siri & Search.

Tap Language to change the language that Siri uses for requests and responses.

Tap Siri Voice to change the voice that Siri speaks.

To hear spoken responses from Siri on your device, tap Settings > Siri & Search > Siri Responses.