Weather is not displayed in the Garmin Connect app so I guess this is a question regarding a specific device and you should rather use the sub forum for that device to get the best help. The general answer is that the temperature unit is changed in the watch settings on the watch.
As e7andy wrote, the setting may differ from model to model, but for example on my Instinct (and probably on many other models), you can change the units for the Weather Widget unsurprisingly in its settings, accessible after long-pressing the Menu button while being on the Weather Widget screen.
