Upgrade from Proxmox 7 to 8

apt update
apt dist-upgrade
pveversion

Check the output of pveversion shows at least 7.4-15

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 Proxmox 8