Hi
I guess you mean that you want to change the weather widget on your home screen. That widget is from the pre installed weather app that came with the phone. Its not really a "default" in the android sense. The "default" widget can be long pressed and then dragged to the REMOVE that appears at the top of the screen
Can then find and install a weather app you like (from Playstore I suggest) and then long press the home screen and select the widget you want (provided by the new app) and drag and drop it onto the home screen. There are lots of apps to choose from, I like Weatherbomb, weather underground, weather & clock widget. There are many others. (You can reinstate the original widget in the same way if you wish)