To change user to www-data:
chown www-data:www-data /var/www

To change all the directories to 775 (-rwxrwxr-x):
find /var/www -type d -exec chmod 775 {} \;

To change all the files to 664 (-rw-rw-r--):
find /var/www -type f -exec chmod 664 {} \;

tout dans une commande
chown -R www-data:www-data /var/www && find /var/www -type d -exec chmod 775 {} \; && find /var/www -type f -exec chmod 664 {} \;