1.  Extract the zip file which you've downloaded from Themeforest.
  2.  Login to your WordPress admin area. Navigate to Appearance > Themes > Add New
  3.  Click on the Upload Theme button on the top.
  4.  Click on Choose File and select "atoz.zip" file from extracted zip file.
  5.  Hit Install Now button.
  6.  Then click on Activate link.

 You can find all about theme installation and setup on  WordPress Сodex as well.

 Note! In case you receive internal server error or "Are you sure you want to do this?" message, you should increase the upload_max_filesize, memory_limit at least to 128MB in php.ini (For Windows platform use php -i | find /i "Configuration File" command to know the location of your php.ini file. Ubuntu command - php -i | grep 'Configuration File'). Most hosting providers have small file upload limit. What you can do in this case: you can upload the theme by FTP or contact your host provider to increase upload limit.