Outline

  1. Backup your system. Typically I create a snapshot.
  2. Update existing packages and reboot the Debian 9.x system.
  3. Edit the file /etc/apt/sources.list using a text editor and replace each instance of stretch with buster.
  4. Update the packages index on Debian Linux, run: sudo apt update
  5. Prepare for the operating system upgrade, run: sudo apt upgrade
  6. Finally, update Debian 9 to Debian 10 buster by running: sudo apt full-upgrade
  7. Reboot the Linux system so that you can boot into Debian 10 Buster
  8. Verify that everything is working correctly.

Source: cyberciti.biz

Step 2. Update installed packages

Type the following apt command or apt-get command:

sudo apt update
sudo apt upgrade
sudo apt full-upgrade
sudo apt --purge autoremove

or

sudo apt-get update
sudo apt-get upgrade
sudo apt-get full-upgrade
sudo apt-get --purge autoremove

Reboot the Debian 9.x stretch to apply the kernel and other updates: sudo reboot

Step 3. Update /etc/apt/sources.list file

sudo cp -v /etc/apt/sources.list /tmp/
sudo cp -rv /etc/apt/sources.list.d/ /tmp/
sudo sed -i 's/stretch/buster/g' /etc/apt/sources.list
sudo sed -i 's/stretch/buster/g' /etc/apt/sources.list.d/*
### see updated file now ###
cat /etc/apt/sources.list

sudo apt update

Step 4. Minimal system upgrade

A two-part process is necessary to avoid the removal of large numbers of packages that you want to keep. Therefore, first run the following:

sudo apt upgrade

Just follow on-screen instructions. During the upgrade process, you may get various questions, like “Do you want to restart the service?” OR “keep or erase config options” and so on. For example, I got a screen as follows indicating new changes.

Press ‘q’ to exit and further you may see additional screens.

Step 5. Upgrading Debian 9 to Debian 10

In addition, minimum upgrades we need to do full upgrades to finish the whole Debian 9 to Debian 10 update process. This is the main part of the upgrade. In other words, execute the following command to perform a complete upgrade of the system, installing the newest available versions of all packages, and resolving all possible dependency:

sudo apt full-upgrade

sudo reboot

Step 6. Verification

It is time to confirm the upgrade. Run:

uname -r
lsb_release -a

Finally, clean up outdated packages using the apt command/apt-get command:

sudo apt --purge autoremove

And there you have it. We have successfully upgraded to Debian Linux 10. Debian project also posted an in-depth guide here that explains other issues one might face during installation.

Debian Official