Hi, the database and FTP are separate parts of a website. Your theme and wordpress files are stayed in your ftp but all the content and theme options saved to database. So, when you update the theme, the theme files can be changed in your ftp. It is simple.
In two words: Download the latest version from ThemeForest. Upload the ZIP file of the theme via wp admin in th eregular way but not activate, because you are using a child theme. Then, go to Appearance > Editor > style.css (this is the child theme file) and change the template value to the new version file name of the theme.