By default gnome came with Fahrenheit as the unit for weather app , Fahrenheit was used as the official temperature scale only in the United States (including its unincorporated territories), its freely associated states in the western Pacific (Palau, the Federated States of Micronesia and the Marshall Islands), the Bahamas, Belize, and the Cayman Island. All other countries in the world now use the Celsius scale.

I am using Fedora and gnome as DE to change Fahrenheit to Celsius in gnome weather app you need  to open terminal then type following command line

Now open gnome weather app again, by now the temperature unit scale will be changed from Fahrenheit to Centigrade .

to change Centigrade to Fahrenheit use this command line

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