The 1Weather app doesn't have a "default" location per se. Whatever location is selected from the list is what will be displayed.
You may add a location from the list of locations.
Launch the app and select the menu, then the pencil icon in the top right to access the edit locations page.
If you need to add a location, press the '+' icon in the top right of the page. Enter a city, ZIP, or postal code to search for a location and then select the location from the results list. Note that you can add a maximum of 12 locations in the app.
You can switch on the follow my location feature for the app to automatically detect your location and show you the weather information for whichever location you travel to.