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