Upgrade from Proxmox Backup Server 2 to 3

apt update
apt dist-upgrade
proxmox-backup-manager versions

Check the output of the last command shows 2.4-2

sed -i 's/bullseye/bookworm/g' /etc/apt/sources.list
sed -i 's/bullseye/bookworm/g' /etc/apt/sources.list.d/pve-no-subscription.list

The commands above switch you to the new Debian Bookworm repos required for the upgrade

apt update
apt dist-upgrade

Once the above is done and clear of errors, restart your Proxmox server

You should now be in PBS 3