Ubuntu

1. Update system: sudo apt update && sudo apt upgrade
2. Remove unnecessary packages and configuration files: sudo apt autoremove --purge
3. Reboot sudo reboot
4. Install update manager (mostly already installed): sudo apt install update-manager-core
5. Check for new Version: sudo do-release-upgrade -c
6. Start upgrade: sudo do-release-upgrade and follow terminal prompts

Debian

1. Update System sudo apt update && sudo apt full-upgrade
2. Reboot sudo reboot
3. Update release package repos: sudo sed -i'.bak' 's/bullseye/bookworm/g' /etc/apt/sources.list (for 11 to 12, replace names with the right versions!)
4. Start upgrade: sudo apt update && sudo apt dist-upgrade
5. Reboot: sudo reboot