You have two options to update the theme:
The easy way is to update the theme via Envato Toolkit plugin – this can be done directly from your Dashboard.
Please navigate to Envato Toolkit page.
Enter your Envato Marketplace Username/Secret API Key on Envato Toolkit.
You will be notified when a new theme version is available for download and will be able to update it automatically from your Dashboard.
The Secret API key can be generated on your Envato account from Settings – Your Details – API Keys.
Download again the theme (installer package) from ThemeForest.
Unzip the theme installer .zip file and you’ll obtain jamsession theme folder.
Connect to your website via FTP and navigate to [wordpress install directory]/wp-content/themes/.
Copy your new jamsession folder over existing folder with the same name, on this location.