Upgrade Debian to new version

Upgrade the packages to the latest version available in your current version.

sudo apt-get update
sudo apt-get upgrade
sudo apt-get full-upgrade

Edit /etc/apt/source.list and replace your current release name with the latest release name ex :

replace  deb https://deb.debian.org/debian/ bullseye main 
by deb https://deb.debian.org/debian/ bookworm main

clean package lists

sudo apt-get clean
sudo apt-get update

Then upgrade your packages to the latest release

sudo apt-get upgrade
sudo apt-get full-upgrade

Clean the packages and reboot

sudo apt-get autoremove
sudo reboot