From https://www.tecmint.com/upgrade-debian-11-to-12/
Migrate
Upgrade to latest packages.
sudo apt update && sudo apt upgrade -y
Purge old packages.
sudo apt --purge autoremove
Update APT
sudo vim /etc/apt/sources.list
/etc/apt/sources.list
deb https://deb.debian.org/debian/ bookworm contrib main non-free non-free-firmware
# deb-src https://deb.debian.org/debian/ bookworm contrib main non-free non-free-firmware
deb https://deb.debian.org/debian/ bookworm-updates contrib main non-free non-free-firmware
# deb-src https://deb.debian.org/debian/ bookworm-updates contrib main non-free non-free-firmware
deb https://deb.debian.org/debian/ bookworm-proposed-updates contrib main non-free non-free-firmware
# deb-src https://deb.debian.org/debian/ bookworm-proposed-updates contrib main non-free non-free-firmware
deb https://deb.debian.org/debian/ bookworm-backports contrib main non-free non-free-firmware
# deb-src https://deb.debian.org/debian/ bookworm-backports contrib main non-free non-free-firmware
deb https://security.debian.org/debian-security/ bookworm-security contrib main non-free non-free-firmware
# deb-src https://security.debian.org/debian-security/ bookworm-security contrib main non-free non-free-firmware
Check if you have some extra sources in /etc/apt/sources.list.d/
ls /etc/apt/sources.list.d/
Remove all sources.list files into /etc/apt/sources.list.d/
sudo rm /etc/apt/sources.list.d/*
Upgrade to Debian 12
sudo apt update
Upgrade packages
sudo apt upgrade --without-new-pkgs
Upgrade distribution
sudo apt full-upgrade
Now you can reinstall all extra sources.list files into /etc/apt/sources.list.d/.
Troubles
Unmet dependencies
sudo dpkg --purge --force-depends libpcre2-posix3
$ sudo apt install libpcre2-posix3 [10:01:27]
Reading package lists... Done
Building dependency tree... Done
Reading state information... Done
You might want to run 'apt --fix-broken install' to correct these.
The following packages have unmet dependencies:
libglib2.0-dev : Depends: libpcre2-dev but it is not going to be installed
libselinux1-dev : Depends: libpcre2-dev but it is not going to be installed
php7.4-dev : Depends: libpcre2-dev (>= 10.30) but it is not going to be installed
php8.0-dev : Depends: libpcre2-dev (>= 10.30) but it is not going to be installed
php8.1-dev : Depends: libpcre2-dev (>= 10.30) but it is not going to be installed
php8.2-dev : Depends: libpcre2-dev (>= 10.30) but it is not going to be installed
E: Unmet dependencies. Try 'apt --fix-broken install' with no packages (or specify a solution).
sudo apt download libpcre2-posix3
sudo dpkg -i --force-overwrite libpcre2-posix3_10.42-1_amd64.deb
sudo apt install libpcre2-dev
sudo apt --fix-broken install
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