EDIT: I've just been looking at the containers list in Portainer and I've noticed I don't have one for nextcloud. I should have this, shouldn't I? All i have is: swag, duckdns, mariadb and portainer. Is that right?
OK, for clarity, and to make things easier for you, I just want to go over what i did last night in detail, so you know exactly what i have done and how. This was all as detailed in the upgrade guide.
I will reference the steps in the guide and what I did. The guide references will be numbered as per the guide itself.
1. Backup. I didn't bother as I have the disk mirrored using rsync
2. Download the latest release. I did this.
3. Stop the webserver. I ssh'd into the server and ran systemctl stop nginx
4. Disable cron-job. I decided that I didn't need to do this as I'd know if I was running a cron-job, but now I wonder that maybe the system would be doing it without me knowing. My fault for not reading this properly. It was 2.00am
5. Rename the existing nextcloud directory. Using the terminal, I navigated to /srv/dev-disk-by-uuid-234ddefe-df8d-4d5e-91f9-8b0375a0d13f/appdata/nextcloud/config/www and then entered the command: mv nextcloud nextcloud-old
6. Unpack the new release downloaded and upload to .../www. I did this by entering the following command: scp -r /Users/nick/Desktop/NCupdate/nextcloud firstname.lastname@example.org:/srv/dev-disk-by-uuid-234ddefe-df8d-4d5e-91f9-8b0375a0d13f/appdata/nextcloud/config/www/
7. Copy the config.php file from the old to the new directly of nextcloud. Here I got lazy, and it was late at night, so I just set up sftp in Filezilla and copied it over there, I was too tired to do it via the command line and didn't want to mess it up. I just downloaded the file locally and then uploaded it to the new location.
8. Copy the data directory to new version if you keep your data directory in /nextcloud. I didn't do this as I didn't see the data directory in the nextcloud directory.
9. 3rd party applications. I didn't bother with this since I don't have and 3rd party application, so far as I know.
10. 3rd party themes. Again, i didn't bother as I don't have any.
11. File ownership and permissions. I got stuck here, this is where I posted to the Nextcloud forum.
After that, I came here.
Since then I've renamed the new nextcloud directory to nextcloud-new, and the old to nextcloud-old one to nextcloud. That was done in Filezilla using sftp. I then started the webserver again with systemctl start nginx
I think that's it apart from the other stuff that I have been asked to do in Portainer.
Now when I navigate to the web instance (blah.duckdns.org/nextcloud) I get a 502.
I hope I'm not being a complete PITA with this. I've been to a few linux-related forums now and this one is by far the friendliest and the only one I can post in without feeling that I'm going to be looked down upon. Aside from all the trouble, I have learned an enormous amount and I am keen to keep that up.