If you buy theme from Themeforest, I suggest you use the Envato Market plugin to install/update theme. It is really convenient, it will tip you which theme update automatically!
The Envato Market plugin lets you view your ThemeForest purchases within the WordPress dashboard and select whether or not to receive updates.
1. Unlike most other free WordPress plugins, the Envato Market plugin isn’t available in the WordPress Plugin Repository. You’ll need to go to GitHub to download it:

2. After you’ve downloaded the plugin, there are a few steps to go through to get the plugin up and running.


Pick up the download file, choose wp-envato-market-master.zip:

3. Activate the plugin, go to “Envato Market”, click the link to get your personal token:

Enter a name whatever, pickup the terms, click the button:

pickup the terms, click the button, copy the code.

4. Go back to your site admin page, paste the token code:

You’ll see the themes what you purchased on Envato. The plugin will now alert you to theme updates – so long as you check the plugin settings regularly. Click on “Update Available” beside each of your themes to automatically update them.

You also could use the plugin to install the theme:
