At first you need to figure out what has changed in the config.php file.
A detailed description of the config.php is shown at the installation guide.
Update from any Version to the latest version
After completing the following checklist, you should have no problems running the latest version.
* Download the latest version * Extract files * Modify config.php * Upload new files and overwrite old files * Run update.php * Select your current version from which the tool should be updated to the latest version. If your version is not listed, you do not need to run that file! * After a successful update you need to delete the update.php file.