Go to your themeforest account and in the download page, re-download the theme (updates are free).
Then go to your admin page > appearance > themes , deactivate and remove current theme and re-upload new version.
Note: You are just replacing theme files, so your setting, customization and contents are safe in the database, but you could also backup your website using WP DB-Manager just in case.
After updating your theme, please hit “Save all options” button in the Option panel because theme needs to re-cache options after each update.
(screencast is also available)
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