To update theme from Themeforest you need to install Envato toolkit plugin.
Unlike most other free WordPress plugins, the toolkit isn’t available in the WordPress Plugin Repository. You’ll need to go to GitHub to download the plugin
After you’ve downloaded the plugin (by clicking the “Download ZIP” click on the right-hand side of the page), there are a few steps to go through to get the plugin up and running.
1. 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.
Generate a free API key on the Themeforest website.
2. Install Envato WordPress Toolkit Plugin
Install and activate the 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. Theme Updates
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.
Envato WordPress Toolkit was released in 2012 and hasn’t changed all that much since. It’s yet to have its own auto-updater, so hopefully this is a feature the folks at Envato will add soon.