First you need to use an FTP client (like FileZila).
You can upgrade your theme by performing the following steps:
1. Download latest All files & documentation file from ThemeForest (your Downloads page)
2. Extract it and locate theme-name.zip
3. Extract theme-name.zip and locate the theme-name folder
4. Copy/Replace the content of theme folder to /wp-content/themes/theme-name folder of your web site.
Using this method, none of existing content or settings will be lost.