can not change theme options

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Warning: Cannot modify header information – headers already sent by (output started at /home/customer/www/rulek.rs/public_html/wp-content/themes/Avada/includes/lib/inc/redux/framework/FusionReduxCore/inc/class.fusionredux_filesystem.php:29) in?/home/customer/www/sitename.rs/public_html/wp-admin/includes/misc.php?on line?1431

Please help me fix this. I am beginner. Where to go to find this problem. I cant change the options everzthing iz freez