You should download the latest installable WP theme from your download page, rename your current theme folder in the wp-content/themes/ folder, unzip the downloaded WP theme from your Themeforest downloads page, and upload it to the wp-content/themes/ folder.
Please note that updating a theme will override any custom changes you make to the parent theme. Hence, we recommend using a child theme. A child theme is not meant to be updated and should contain only your custom changes.