Cannot modify header information

Hello! I just bought a Pop Rock Pro theme and was trying to upgrade it (first I tryied it for free), but I am now very unsatisfied with it. I can't import demo nor can I make customize the theme. It is giving the same message all the time: Warning: Cannot modify header information - headers already sent by (output started at /home/customer/www/astridnolvak.com/public_html/wp-content/themes/pop-rock-pro/inc/custom-header.php:1) in /home/customer/www/astridnolvak.com/public_html/wp-includes/functions.php on line 6221 Can you help me out, please? With regards, Agnes