You need to update your apt sources file. Here is the process. Run the following commands as root. Use sudo -s first to avoid having to prefix every command with sudo. This assumes you are running bullseye. If not you need to update to bullseye first.

Update your current installation first:
apt udpate
apt full-upgrade -y

Then upgrade:
sed -i 's/bullseye/bookworm/g' /etc/apt/sources.list
export LC_ALL=C
apt update
apt upgrade --without-new-pkgs -y
apt full-upgrade -y

Then reboot.