After upgrade from Debian 10 Buster to Debian 11 Bullseye security updates 404 not found

Doing recommended upgrade path, TL;DR:

Backup data, then

apt update
apt upgrade
apt dist-upgrade
reboot

Also disable all non main stream packages if any.

Then preparing APT for new distribution:

sudo sed -i 's/buster/bullseye/g' /etc/apt/sources.list
sudo sed -i 's/buster/bullseye/g' /etc/apt/sources.list.d/*

The first command shows error:

apt update
...
Err:6 http://security.debian.org/debian-security bullseye/updates Release
  404  Not Found
Reading package lists... Done
E: The repository 'http://security.debian.org/debian-security bullseye/updates Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

This is regarding change in Debian 11 repository structure. RTFM requires time, so bump answer here:

For Debian bullseye, the security suite is now named bullseye-security instead of codename/updates and users should adapt their APT source-list files accordingly when upgrading.

Thus execute one more command:

sed -i 's/bullseye\/updates/bullseye-security/g' /etc/apt/sources.list

Now apt update works great again, and you can continue with upgrade:

sudo apt update
sudo apt upgrade --without-new-pkgs
sudo apt full-upgrade
reboot

Check if system is upgraded:

lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description:    Debian GNU/Linux 11 (bullseye)
Release:    11
Codename:   bullseye

After successful reboot, you can remove old kernel:

sudo apt purge linux-image-4.19*

Also review log files, for error messages of conflicting configurations or other issues.

Issue list is here: https://www.debian.org/releases/bullseye/amd64/release-notes/ch-information.html

If you followed the various Debian update tutorials out there, chances are you ended up with an incorrect /etc/apt/source.list

As most "fixes" are listed sed command patches, they may not work if your original source.list content isn't a perfect match.

In that case, just edit source.list manually, the new bullseye security lines should read like

deb http://security.debian.org/debian-security/ bullseye-security main
deb-src http://security.debian.org/debian-security/ bullseye-security main

or alternatively if you do not need source and want contrib, it could be just

deb http://security.debian.org/debian-security/ bullseye-security main contrib