You’re very welcome…. you won’t have to update the child theme if you are using that; just update the parent theme of Circumference. However, if you made modifications to the actual Circumference theme and it’s style.css, then it won’t be easy to update. If you did not make any changes to theme or it’s style.css file, then you would download the updated version, unzip it, but also unzip the circumference.zip (the actual theme) to your desktop and using FTP, you would upload and replace the “circumference” theme folder with the new one (the one with the theme files in it such as the style.css file).
IMPORTANT: Make sure you create a backup before updating; always best to play safe. In fact, I strongly recommend you have a backup plugin to make regular backups of your site…especially when updating WordPress and plugins, because as I said, it’s best to play safe whenever you update something…not just a theme.