1.编辑/etc/apt/sources.ist.d/pve-enterprise.list

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

注释

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

2.添加免订阅的官方更新

echo "deb http://download.proxmox.com/debian/pve stretch pve-no-subscription" > /etc/apt/sources.list.d/pve-install-repo.list
wget http://download.proxmox.com/debian/proxmox-ve-release-5.x.gpg -O /etc/apt/trusted.gpg.d/proxmox-ve-release-5.x.gpg
最后执行更新
apt update && apt dist-upgrade

顺利升级到5.3