Please do the following the steps here :
1. Login to Themeforest, go to your dashboard and click on “My Settings.” The API Keys screen allows you to generate a free API key.
2. Install and activate the Envato Toolkit plugin. A new menu item, “Envato Toolkit,” will appear in the admin sidebar. Click on the sidebar link and enter your marketplace username and secret API key. Click “Save Settings.”
All of your theme purchases will now appear on this page after it refreshes.
3. The plugin will now alert you to theme updates – so long as you check the toolkit settings regularly.
Click on “install automatically” beside each of your themes to automatically update them.
The toolkit settings page also allows you to install themes that you haven’t already uploaded to your site, delete ones you don’t use and view version details. You can also set up theme backups.
It’s a fairly simple plugin, but also a helpful one.