To update the themes, you can simply use the same way how you install the themes.
- Firstly, you will need to download the themes from Themeforest.
- After that you will just need to follow how you install the themes you previously do, using WordPress themes uploader or using FTP uploader.
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.
For more detail about how to use envato toolkit, please follow this url.