Cannot Modify Header Information

Hello, After reading lots about this message, I have learnt that I am required to change some coding in the relevant file but I am not sure what I am meant to be changing. I have the following code:

Warning: Cannot modify header information – headers already sent by (output started at /home/peekaboo1949/public_html/www.llanddewcc.org/wp-content/plugins/drop-shadow-boxes/freemius/includes/entities/class-fs-payment.php:1) in /home/peekaboo1949/public_html/www.llanddewcc.org/wp-login.php on line 394

Warning: Cannot modify header information – headers already sent by (output started at /home/peekaboo1949/public_html/www.llanddewcc.org/wp-content/plugins/drop-shadow-boxes/freemius/includes/entities/class-fs-payment.php:1) in /home/peekaboo1949/public_html/www.llanddewcc.org/wp-login.php on line 407

From my understanding I have to change code on line 394 and 407 of the wp-login.php, which I have downloaded. The two lines of code are:

header(‘Content-Type: ‘.get_bloginfo(‘html_type’).’; charset=’.get_bloginfo(‘charset’));

AND

setcookie( TEST_COOKIE, ‘WP Cookie check’, 0, COOKIEPATH, COOKIE_DOMAIN, $secure );

When I access the website at https://www.llanddewcc.org/wp-login.php, I can see it from the front end but when I try login into the wordpress dashboard I get the following message:

ERROR: Cookies are blocked due to unexpected output. For help, please see this documentation or try the support forums.

Any help greatly appreciated.

Charlotte