You can redownload the theme in your downloaded items tab on themeforest and update via ftp (upload to wp-content/themes/ folder)

Or simply use the envato toolkit plugin to automatically update the theme!

All you need is to include your Envato username and an API key.

You will need to generate an API key to link your WordPress site to your Themeforest account.

Login to Themeforest, go to your dashboard and click on “My Settings.” The API Keys screen allows you to generate a free API key.