This is the way to update theme by a plugin of envato. Do follow these steps to update by Envato Toolkit.

Step 1. Download Envato WordPress Toolkit here.

Step 2. Install and active the plugin in Dashboard >> Plugins >> Add new.

Step 3. Get your API keys.

Step 4. Dashboard >> Envato Toolkit >> input API keys (Marketplace Username &  Secret API Key).

Step 5. After input API keys, there will be “Themes” tab. Now you can update theme automatically.