There is a setting for the temperature scale in your Location. I don’t know why you can’t edit it from the SmartThings app but can from the Classic app. You should be able to just login to the Classic app and find everything already setup but I prefer to login to the IDE at account.smartthings.com instead.
If you login to the IDE, and choose ‘Location’ (‘My Location’ on bigger desktop windows) from the menu, you will see your Locations listed. Choose the one of interest and you’ll get a page showing your location details including the temperature scale. There is an Edit button that allows you to edit some of the information, including temperature scale.
I should note that at the moment (5am in the UK), account.smartthings.com temporarily isn’t responding for me.