All themes come with a changelog.txt file under main theme files which includes data of what files are added/changed.
To update your theme please go to your ThemeForest account and download the respective theme files. After downloading new theme files you can either replace only files needed as mentioned in changelog.txt or you could simply replace all files via FTP on your server. Remember that replacing all files may lead in losing any manual customization made.
Or you can just use the Envato Market WordPress plugin by Envato to update your theme automatically.
It’s recommended to always use a child theme so you can easily make customization and whenever there is an update you can update without any hustle.