How can I change from Centigrade to Fahrenheit on Watch?

See if this works:

To change to Fahrenheit on your Apple Watch:

On your iPhone, in the Weather app, tap on °C / °F to switch between units.

The change will be reflected automatically in the Weather app (and its watch face complications) on your Apple Watch.

Credit to Jonathan UK for this answer.

From within the Weather app on your phone, Click on the three lines in the lower right corner of the screen.

On the next page (where you can choose what location), click on the 3 dots in the upper right-hand corner of the screen. You will find that you can choose between Celsius and Fahrenheit there.

I believe that your watch will follow suit.

I hope that this helps you out ;-)