First make a backup of your WordPress database in case something goes wrong.
1. Download the latest version from Themeforest and unzip it
2. Log in to your server using an FTP client and navigate to your WordPress installation: wp-content - themes
3. Delete (or just rename) your old theme folder on server
4. Copy the updated version theme folder to the server
If you are not comfortable with FTP, login to your WordPress admin dashboard and go to Appearance - Theme Options - Theme Upgrade. You will need to input your Themeforest API key.
To generate an API key, select Settings from the account dropdown, then navigate to the API Keys tab. Multiple API keys can be generated so it is recommended to use one per application.