This method is now deprecated, to read how you can update your theme please read this article.
(Deprecated) Update using Envato WordPress Toolkit
In your WordPress admin sidebar you should see a section called Envato Toolkit.
If you don’t see it go to Appearance > Install Plugins Install and activate the toolkit plugin. A new menu item, Envato Toolkit will appear in the admin sidebar.
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.
Click Envato Toolkit on the sidebar and on Marketplace Username box write your themeforest username and on Secret API Key paste the code you generated.
All of your theme purchases will now appear on this page after it refreshes, and you’ll see here all available updates, always at one click away
That’s it 🙂
How do I update the premium plugins?
Just go to Appearance > Install Plugins and install the latest version of that plugin.