Since I don't know what widget is is, I can only guess. But here are a couple of things to try.
The widget probably takes that setting from the underlying weather app that feeds it data. If you tap on the widget, it should take you to that app, where you can adjust settings.
Another possibility is that the widget has it's own settings, although this seems less likely. But if the first option doesn't work, try deleting the widget from the screen, and re-adding it. Generally, Widgets that have configuration options prompt you to set them when you add the widget to the screen.
HTH.