Updating the theme is easy.
1. Download the theme again from ThemeForest to your computer.
2. Unzip the file that you downloaded from ThemeForest.
3. In the WordPress folder unzip the file named theme-mingle-v1.6.3.zip (or whatever the latest version of the theme happens to be named).
4. Using FTP, upload the contents of the parallelus-mingle folder into the wp-content/themes/parallelus-mingle/ folder on your server, overwriting the files and folders of the same names that are already there.
(Alternatively, delete the old “parallelus-mingle” on your server and then just upload the new “parallelus-mingle” from your computer to your server.)
That’s all there is to it. Your content and your settings are totally unaffected by this process because they are stored separately in your site’s database.