Update system
sudo apt update sudo apt upgrade sudo apt full-upgrade sudo apt --purge autoremove
Reboot to reload any kernels
sudo shutdown -r 0
Update sources to buster w/ sed search and replace:
sudo sed -i 's/stretch/buster/g' /etc/apt/sources.list sudo sed -i 's/stretch/buster/g' /etc/apt/sources.list.d/*
Verify
cat /etc/apt/sources.list
Update and upgrade
sudo apt-get update sudo apt-get upgrade sudo apt full-upgrade