Nextcloud released update 12.0.1.(Aug 7 2017) and if you are using package from my repository (jessie or stretch) You can make very fast upgrade. I will demonstrate how to upgrade from 12.0.0 to 12.0.1. in this short guide.

Check standard changelog before upgrade on link and blog This update brings many improvements and strongly recommend that you install the last update.

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 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

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

* 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 12.0.1 !!!