Point to note: If you did any changes on theme files, then keep backup of them, so that you can upload again after updating the theme.
Theme Update Automatically
You can update the theme automatically by following this link– https://envato.com/market-plugin/
Theme Update manually
Download the latest package from ThemeForest, unpack the files, and upload the theme on the server.
You need to upload the theme on your server by following these steps–
1) Login to your cPanel
2) Go to public_html > wp-content > themes
3) Upload the updated zip file of your theme
4) Delete the theme of the present version
5) unzip the updated version of the theme
6) After unzip, you may delete the theme zip