Update all stuff first
  apt-get update
  apt-get upgrade
  apt-get dist-upgrade
Upgrade
  sed -i 's/stretch/buster/g' /etc/apt/sources.list
  sed -i 's/stretch/buster/g' /etc/apt/sources.list.d/pve-install-repo.list
  apt-get update
  apt-get upgrade
  apt-get dist-upgrade
Optional clean old kernels
  dpkg --list|grep pve-kernel
  apt purge pve-kernel-4.15.18-29-pve