HELP HELP HELP!!!

My web is https://battletime.org/
andd when i go to https://battletime.org/wp-admin i cannot i get this message:

Warning: Cannot modify header information – headers already sent by (output started at /home/battleti/public_html/wp-content/themes/FashionStyle/functions.php:364) in /home/battleti/public_html/wp-includes/pluggable.php on line 865

if anybody can help me with this.

Im not advance user so please explane to me step buy step if you know.

Thank you all in advance i hope that somebody know how to fix this cuz i dont even know what i did.