Note that any changes you made to your theme will be lost if you did not make them through a child theme, which was included in the download. Do not overwrite your existing child theme. Only update the parent/main theme.
Updating your theme is easy! First you'll need to download the latest version of your theme from your ThemeForest downloads page.
Once you have the latest version you can upload it one of two ways:
Simply go to your wp-content/themes folder and drop the latest version of the theme in there, overwriting the existing theme.
Via the WP Dashboard
You can upload the themename-theme-only.zip through your WP dashboard. But first, you need to delete the current version of the theme under Appearance > Themes. Don't worry, your settings will remain intact. To do this, you'll need to activate a different theme (unless you're using a child theme), then click Theme Details followed by the delete link on the theme you want to update.
Note: DO NOT DELETE YOUR CHILD THEME.
Once that is done, navigate to Appearance > Themes and then click on the Add New button followed by the Upload Theme button. Then follow the on-screen instructions from there.