Unfortunately there is currently no button for resetting the settings back to default. However, you can do that by deleting settings folder located in C:\Users\YOUR USER NAME\AppData\Local\NineToFive\Saved\Config
Please notice that the AppData folder is hidden. The easiest way to access the hidden AppData folder is by pressing the windows button on your keyboard, typing run and pressing enter. Then typing %AppData% on the screen that pops-up and pressing enter should bring you directly to the hidden folder.