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.