Although you are posting to the forum of the Garmin Connect Mobile Android app, you probably mean on the watch, since there is no weather on the app. Since you did not tell what watch you own, it is diffiicult to advise specifically, but typically you change the units in the settings of the widget - open the Weather widget on your watch and enter its settings (on my watch I have to long-press the Menu button - check the user manual of your model, if you do not know how to do it on your device).
