The 1Weather app doesn't have a "default" location per se. Whatever location is selected from the list is what will be displayed.
yYou can add a location as "My Location" that will then update based on actual current location.
To add "My Location," Tap the 1Weather icon at the upper left to access the location list and select "Add Location."
Tap the GPS icon in the upper right and allow the app to automatically detect your location and then tap the returned location to add it as MyLocation.
This should then update based on your current location as long as the follow my location option is turned on in settings.
You can also set the notification to "My Location" by going into setting and tapping "current conditions" (the words, not the slider) and selecting the "Location" item to select what location you want shown in the notification.