As for the user-permissions issue mentioned by the OP, there's a fairly simple solution: use a live-cd from your distro (I assume Devuan in this case). Boot your system into the live-cd modus and start a terminal. Then, assuming your entire system is on one SATA disk in a single partition (sda1):

mkdir /tmp/devuan-rescue
mount /dev/sda1 /mnt/devuan-rescue
cd /tmp/devuan-rescue
chown -R root:root ./*

This creates a temp. directory, then mounts the entire system in that and changes the ownership of all files to root as part of the group root. That should allow you to log back in (as root, obviously) into the system and restore file permissions from there. See also

man chown

This'll list you the various options and correct syntax of the command.

give "www-data" permissions to Nextcloud but it was given to the whole system, the issue is that I have no idea how to fix that.

Just to be clear did you change the permissions for all files in the operating system or just the files of the nextcloud install?  If you did, then I would suggest reinstalling the operating system.

What I do is to make the <user> the files owner and then set "www-data" as the group owner.  Then I add the <user> to the "www-data" group.  Finally, I change the chmod of the files to 0775 so that both the files owner and www-data group have read and write ability of the files.  To do this, I only have a single user system with no other user accounts. 

I would be interested in somebody else adding their input on a proper way to set the permissions in a multi-user system.  I have spent some time searching the web for similar information but what I have found mentioned is somewhat conflicting.  Some say to make the user "www-data" the owner of the files, but I am unsure about that.

I was trying to install as indicated, but in one step I ran to give "www-data" permissions to Nextcloud but it was given to the whole system, the issue is that I have no idea how to fix that.

At this moment I can't do anything because I don't have permits for that same reason.

Any comments?

but should be

The web updater has improved a lot over the past few versions.  If I may say so, please remember to back up your NC install (files, data, and database) before updating as it may have mishaps.  Good luck with it!

I am in need of your help, I have a VPS with GNU / LINUX DEVUAN, I need to install the Nexcloud, will you have any idea how to do it? Can you guide me?

Thank you very much!

Best regards,
Brian.