1. Please backup your current theme. To avoid replaced your custom code.
2. Download newest version: http://themeforest.net/downloads
3. Extract the main zip file, then extract the theme zip.
4.1. Remove your older theme and replace all with the new version
4.2. Or you can read the changelog.txt and change the changed files only.
5. Restore your own custom code from backup file to the newest theme.
6. Use child theme instead for save your custom code.