How do I switch the weather app to celsius?

System Settings > General > Language & Region and change the "temperature" setting to celsius.

Old, but try this: https://forums.macrumors.com/threads/how-can-i-change-the-weather-widget-to-celsius.2296978/

BaruchZed13 wrote:

Thank you!

you're welcome.

Wow what a silly place for that to be.

not really. the temp and measurement values are usually determined by where on the planet it is that each of us live. ;)