# vi /etc/apt/sources.list
deb http://ftp.debian.org/debian stretch main contrib
deb http://ftp.debian.org/debian stretch-updates main contrib
deb http://ftp.hk.debian.org/debian stretch main contrib
deb http://download.proxmox.com/debian/pve stretch pve-no-subscription
deb http://ftp.hk.debian.org/debian stretch-updates main contrib
# security updates
deb http://security.debian.org stretch/updates main contrib
最后:
apt update && apt dist-upgrade