On my iPhone, on the app, I finally found settings (icon under the Home iconas I recently learned). I then scrolled down (also not obvious). I changed from C to F and it automatically asked me to log-in, which I did and the switch stuck. I know I am forgetting something, but the key, in addition to being logged in (on the app on your iPhone!), is to scroll all the way down in Settings to find Save (in red, not grayed out). I did first log into the website, but noticed it was already set to F. Not sure if just then logging into the app made the difference (since settings are supposedly in the cloud) or if actual saving on the app while logged into the app did it.