Plugins are able to make a self update from central repository. When accessing “Plugins” menu you can see which plugins have available update. Update workflow is exactly the same as updating regular plugins from WordPress.org but you need to insert your purchase code into the field under the plugin name.
Automatic updates are available only for plugins. Theme does not have automatic updates.
Your purchase code is available on ThemeForest. You can found it by accessing “Downloads - Superlist - Download - License certificate & purchase code (text)”. After the save, WordPress will be able to access plugins ZIP files. If you make a mistake and the purchase code is for example from wrong theme, you can visit “Customizer - Horizon General” and change or remove the purchase code.
It is recommended to not edit the plugins files. You can change all the functionality from child theme.