I had this when trying to update/upgrade buster packages.
First, I had to update /etc/sources.list to use the archive paths:
deb http://archive.debian.org/debian buster main
deb http://archive.debian.org/debian-security buster/updates main
deb http://archive.debian.org/debian-volatile buster/volatile main
deb http://archive.debian.org/backports.org buster-backports main
Then the usual steps:
sudo apt-get update
sudo apt-get upgrade
Version upgrade
To get the latest version (bullseye) I updated /etc/apt/sources.list to:
deb http://deb.debian.org/debian bullseye main
deb-src http://deb.debian.org/debian bullseye main
deb http://deb.debian.org/debian bullseye-updates main
deb-src http://deb.debian.org/debian bullseye-updates main
deb http://security.debian.org/debian-security/ bullseye-security main
deb-src http://security.debian.org/debian-security/ bullseye-security main
and repeat but with the full-upgrade option:
sudo apt-get update
sudo apt-get full-upgrade
xscreensaver warning
If during the upgrade it says:
One or more running instances of xscreensaver or xlockmore have been detected on this system. Because of incompatible library changes, the upgrade of the GNU libc library will leave you unable to authenticate to these programs. You should arrange for these programs to be restarted or stopped before continuing this upgrade, to avoid locking your users out of their current sessions.
then use these steps:
pgrep -l screensaver
pkill screensaver
pgrep -l screensaver