This is done by deleting the configuration file called settings.cfg. This will force the game to use the predefined defaults within the executables.
Double click on the config_directory.bat. This will take you to the directory where the config-file is stored. Delete the settings.cfg file.
The config-file is stored at ~/.teeworlds/settings.cfg. Just run this command from a terminal to delete it.
# rm ~/.teeworlds/settings.cfg
Delete settings.cfg in ~/Library/Application Support/Teeworlds.