I noticed that I was not able to see the toggle to change between F and C when I had both the gear and the galaxy plugin installed. Once I uninstalled the galaxy plug in (keeping the gear plug in, I was able to see the toggle to change to F. Now I have BT connectivity problems that I can’t figure out, but I do have F on my unconnected Watch. ;-)
You should already have the Samsung Wearable app installed on your phone (used to pair your phone to your watch), open it, scroll down to Apps (Manage, Reorder, Uninstall will be displayed underneath it), then scroll down to Weather and click the cog to the right. One of the first things you can change is the Unit.
Been struggling with this since yesterday as well. Hope it helps.