You can downgrade by downloading an old firmware from http://www.sammobile.com/ but its not recommended to do so. it'll cause problems to your phone like losing the IMEI code. Why do you want to downgrade ?
Samsung Galaxy S3 GT-I9300 International (MID-IRAQ)
XDA Developers was founded by developers, for developers. It is now a valuable resource for people who want to make the most of their mobile devices, from customizing the look and feel to adding new functionality. Are you a developer?