Error Message> can't get to admin page

I am getting several messages:

Warning: Cannot modify header information – headers already sent by (output started at /home/content/41/7579341/html/wp-content/plugins/wp-slimbox2/index.php:1) in /home/content/41/7579341/html/wp-includes/pluggable.php on line 875

All are the same with a different final number. I cannot get to my admin pages. This occurred right after I installed the newest version 3.6.

What do I do? Thanks.