How did you get it to show c by default?
Full marks to someone. Bixby now defaults to Centigrade for me, finally. Well done Samsung.
My Samsung watch reports in Centigrade, as does bixby now so it has to be a software adjustment rather than a hardware one. I suspect your problem will be to find someone who knows how to make the adjustment. Good luck. If you keep asking Google it's highly likely someone has a solution.