From LimeSurvey Manual
To modify the configuration settings of the installation, you have to edit the optional settings. They can be found in the /application/config/config-defaults.php, which is located in the LimeSurvey root directory. The default settings of the standard installation can be found in config-defaults.php. Some of them can be overridden by using the global settings dialog, while the others will have to be manually edited.