Best if important stuff is backed up first and custom apt sources disabled.

• apt update
• apt upgrade
• cp /etc/apt/sources.list /etc/apt/sources.list.orig
• sed -i 's/stretch/buster/g' /etc/apt/sources.list
• diff sources.list.orig sources.list to verify the change
• To kick off the upgrade, apt update && apt upgrade
• Finally, apt dist-upgrade
• reboot
• after reboot, verify with lsb_release -a