Settings::setExtConf() 

\nn\t3::Settings()->setExtConf($extName = '', $key = '', $value = ''); 

Write extension configuration. Writes an extension configuration in the LocalConfiguration.php. The values can also be corresponding configuration in ext_conf_template.txt, the values can also be edited via the Extension Manager / the Extension configuration in the backend.

\nn\t3::Settings()->setExtConf( 'extname', 'key', 'value' );

| @return mixed

public function setExtConf( $extName = '', $key = '', $value = '' )
{
	$coreConfigurationManager = GeneralUtility::makeInstance(\TYPO3\CMS\Core\Configuration\ConfigurationManager::class);
	$result = $coreConfigurationManager->setLocalConfigurationValueByPath("EXTENSIONS/{$extName}/{$key}", $value);
	return $result;
}