Fatal error on logging in

I get the following error when I try and log in after my file has loaded. I did change the theme so I guess the file has been corrupted? How do I de-install and then re-install? It was loaded automatically through my server host LCN

Fatal error: Cannot redeclare win_is_writable() (previously declared in /www/sites/d53/c13/www.360leadership.co/web/wp-includes/functions.php:1433) in /www/sites/d53/c13/www.360leadership.co/web/wp-admin/includes/misc.php on line 554