Change the Celsius to Fahrenheit

What about the Weather app on your iPhone. Is the temperature set to Celsius (settings found on bottom-right)? I know, I continue to point you to your iPhone. I think that the HomePod is somehow synced with your Regional settings of your iPhone.

I own a HomePod. My regional settings on my iPhone are set to USA, with the Temperature Unit set to Celsius. When I ask the weather forecast my HomePod answers me with temperature in Celsius.

You may try this: Hey Siri, What is the temperature in Fahrenheit ?

Try to switch to the Metric System in Settings -> General -> Language & Region -> Temperature Unit on your iPhone.