Try turning off plugins to see if they cause the issue. Also, when on the WordPress Updates page, try pressing the Check Again button to see if this could clear the theme update notice. This might also be caused by your server which could be caching the old files.
Note: Not using a child theme or Custom CSS option will result in you losing your modifications when you update the theme. https://ronangelo.com/forums/topic/customizing-wordpress-themes/