You have two options to update the theme:
The easy way is to update the theme via Envato Market plugin – this can be done directly from your Dashboard.
Please check this resource for more details on how to update your theme using Envato Market plugin.
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.
After updating the theme, please check if any of the plugins that are part of the theme package needs to be updated. Please navigate to Appearance – Install JamSession Plugins and update all the plugins that are part of the theme package.