If you don't want to pay for a Proxmox subscription you can still get updates through the no-subscription channel.

cd /etc/apt/sources.list.d
cp pve-enterprise.list pve-no-subscription.list
nano pve-no-subscription.list

Edit the pve-no-subscription.list to the below

deb http://download.proxmox.com/debian/pve bookworm pve-no-subscription

Run updates, but only use dist-upgrade and not regular upgrade as it may break dependencies.

apt-get update
apt-get dist-upgrade