Installing the new version of theme, almost the same as installing new themes. You can update your theme by downloading a fresh copy from ThemeForest, deleting the current version you have on your server and uploading the new version. For more information about installing themes, please follow this url.
Updating Required Plugins
Installing the new version of required plugin, almost the same as installing plugin previously. You can update the required plugin by remove the current version you have on your server and then go to Appearance→Install Plugins to install the newer version.
Envato provide plugin that enable automatic update for themes. You can download those plugin right here . You will just need to input your API key.