When new update is available, the ThemeForest will notify you that you can download new packages and install it manually. This is not the recommended approach. Using automatic updates is more efficient.
To access automatic updates you can make us of the Envato WordPress Toolkit plugin. If you haven't installed it yet, you can do that through WordPress Admin › Appearance › Install Plugins panel.
For more information about its configuration, you can read the Envato WordPress Toolkit Guide For Automatic Theme Updates