How can I update the Magento core config value
Its using same method as add magento core config.
$coreConfig = Mage::getModel('core/config');
$scope = "default";
$scopeId = 0;
$coreConfig->saveConfig('sectionName/groupName/fieldName', 'your value', $scope, $scopeId);
How To Add A Magento Store Config Value Programmatically