Verify what needs upgrading

Run

lsb_release -a

and verify the currently installed version. In my case, “Debian GNU/Linux 9.13 (stretch)” At this point we have decided that we are upgrading. Backup anything on the system that you care about.

Prepare the system

I personally like the make sure everything is up to date before we start changing things.

sudo apt-get update
sudo apt-get upgrade

Update references

Update the references to the apt repositories sudo vim /etc/apt/sources.list stretch => buster

Upgrade the packages

sudo apt-get update
sudo apt-get upgrade

Upgrade Debian itself

sudo apt-get full-upgrade

Reboot so the new kernel loads

sudo reboot

Verify the change

lsb_release -a

Simple, simple. If there were any issues, the easiest way to proceed is likely a fresh install and restore from backup.