I first started out with

sudo apt update && sudo apt upgrade

and then I got

On KDE neon you should use `pkcon update` to install updates.
If you absolutely must use apt you do have to use dist-upgrade or full-upgrade in place of the upgrade command.
https://neon.kde.org/faq#command-to-update
Abort.

So I went ahead and typed `sudo pkcon update` and got things like

E: http://archive.neon.kde.org/user focal/main amd64 plasma-discover-common all 5.26.0-0xneon+20.04+focal+release+build68 is not (yet) available (Unable to connect 127.0.0.1：20172：)

I've set `Acquire::Proxy::http "none"` in `/etc/apt/apt.conf` and in system settings GUI I've reset the proxy settings, the environment variables for proxies are all empty, but the problem persists. I do have a proxy server running in the background and I set proxy in GUI before. Now I can set a new proxy address in GUI, which means the proxy settings used by pkcon can be updated, but I can't unset it. I'm running out of ideas now.
Apology for my poor English.