1. Automatically : You can use Envato Market plugin to update the theme automatically. Instructions here : https://ibndawood.com/how-to-update-a-themeforest-wordpress-theme-or-a-codecanyon-wordpress-plugin/
2. Semi-Automatically : Sometimes (1) does not work. You should install this plugin : https://wordpress.org/plugins/easy-theme-and-plugin-upgrades/ , download the latest installable WP theme zip file from your Themeforest downloads page and then upload it via Appearance > Themes > Add New > Upload Theme and upload the zip file. The “easy upgrades” plugin will backup the old theme and install the new one.
3. Manually : You should download the latest installable WP theme from your themeforest downloads page, rename your current theme folder in wp-content/themes folder, unzip the downloaded WP theme from your Themeforest downloads page and upload it to wp-content/themes folder.
Please note that updating a theme will override any custom changes that 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.