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.