The support was a little vague on the Nextcloud site.
Here are some more specific directions:
1) First you want to make a copy/backup of your Owncloud folder.
sudo cp -r /var/www/owncloud /var/www/owncloud.old
2) Then remove everything inside your owncloud folder.
sudo rm -rf /var/www/owncloud/*
sudo rm -rf /var/www/owncloud/.*
3) The reason we made a backup of the Owncloud folder was to make sure we still
have the config and data, so cp that in to the owncloud folder again
sudo cp -r /var/www/owncloud.old/config /var/www/owncloud/
sudo cp -r /var/www/owncloud.old/data /var/www/owncloud/
sudo cp /var/www/owncloud.old/.htaccess /var/www/owncloud/
sudo cp /var/www/owncloud.old/.user.ini /var/www/owncloud/
4) Install Nextcloud
We are going to download nextcloud, make sure you get the latest release
5) Extract the tar ball and move it to Apache’s web root.
tar -vxjf nextcloud-9.0.50.tar.bz2
mv * /var/www/owncloud/
6) Change the permissions while in the owncloud directory(it now has newcloud files inside of it):
wwwrun:www * -R or chown www-data
7) The last thing we will need to do is the upgrade to Nextcloud.
sudo -u www-data php occ upgrade
And that should be it! open up your browser and enter your IP/FQDN to confirm it works!
I did copy most of this information from:
They were good posts, but missing some commands. Hopefully this helps!