Proxmox 8 и выше:

nano /etc/apt/sources.list.d/pve-enterprise.list
#deb https://enterprise.proxmox.com/debian/pve bookworm enterprise
deb http://download.proxmox.com/debian/pve bookworm pve-no-subscription
nano /etc/apt/sources.list.d/ceph.list
#deb https://enterprise.proxmox.com/debian/ceph-quincy bookworm enterprise
deb http://download.proxmox.com/debian/ceph-quincy bookworm no-subscription

После обновляем:

apt update && apt upgrade -y

Proxmox 7 и ниже:

nano /etc/apt/sources.list

Добавить строку в конец:

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

И еще:

nano /etc/apt/sources.list.d/pve-enterprise.list

Закомментировать строку:

#deb https://enterprise.proxmox.com/debian/pve bullseye pve-enterprise

После обновляем:

apt update && apt upgrade -y