You can manually update the themes bought from ThemeForest with either of the method explains as below –

Geeky Method –

  1. Log into FTP Account using FileZilla or similar program.
  2. Go to wp-content/themes directory
  3. Move to the theme directory that you want to update.
  4. Now, download the latest version of theme from ThemeForest.net/downloads
  5. If you have downloaded the zip, extract it. Find the theme folder. It generally has style.css file in it.
  6. Upload all files in that folder to the directory on FTP we found in step #3 above.

Here’s a similar video that will give you an idea.

User friendly Method –

  1. Download Envato Toolkit plugin from here.
  2. Install the above plugin
  3. Required API Key can be generated as explained here.
  4. This plugin will allow you to update themes you’ve bought from ThemeForest.

You can read another article on this topic here.