Nextcloud released 12.0.0.(May 22 2017) and if you are using package from "Jurisic repository". I will demonstrate how to upgrade from 11.0.3 to 12.0.0. in this short guide.

Check standard changelog before upgrade on link https://nextcloud.com/changelog/ and blog https://nextcloud.com/blog/welcome-to-nextcloud-1.

Upgrade is very simple (open console as root):

apt-get update
apt-get upgrade 

sudo -u www-data php /var/www/nextcloud/occ upgrade
sudo -u www-data php /var/www/nextcloud/occ maintenance:mode --off

sudo -u www-data php /var/www/nextcloud/occ app:enable contacts
sudo -u www-data php /var/www/nextcloud/occ app:enable calendar
sudo -u www-data php /var/www/nextcloud/occ app:enable tasks
sudo -u www-data php /var/www/nextcloud/occ app:enable spreed

* When Nextcloud going in maintenance mode automatic all app will be tourn off, and in my case I need to enable "contacts,calendar,tasks and spreed", if You using another app enable with "sudo -u www-data php /var/www/nextcloud/occ app:enable app-name".

That is all, enjoy in Nextcloud 12.0.0 !!!