Upgrade your Raspbian install from Stretch to Buster. This is basically the same procedure as upgrading Jessie to Stretch that I covered previously.

Prepare

Get up to date.

$ sudo apt-get update && sudo apt-get upgrade -y

Verify nothing is wrong. Verify no errors are reported after each command. Fix as required (you’re on your own here!).

$ dpkg -C
$ apt-mark showhold

Prepare apt-get Sources

Update the sources to apt-get. This replaces “stretch” with “buster” in the repository locations giving apt-get access to the new version’s binaries.

$ sudo sed -i 's/stretch/buster/g' /etc/apt/sources.list    
$ sudo sed -i 's/stretch/buster/g' /etc/apt/sources.list.d/raspi.list    

Verify this caught them all by running the following, expecting no output. If the command returns anything having previously run the sed commands above, it means more files may need tweaking. Run the sed command for each. The aim is to replace all instances of “stretch”.

$ grep -lnr stretch /etc/apt    

Speed up subsequent steps by removing the list change package.

$ sudo apt-get remove apt-listchanges

Do the Upgrade

To update existing packages without updating kernel modules or removing packages, run the following.

$ sudo apt-get update && sudo apt-get upgrade -y

Alternatively, to include kernel modules and removing packages if required, run the following (choose one, not both. See this question for details).

$ sudo apt-get update && sudo apt-get full-upgrade -y

Cleanup old outdated packages.

$ sudo apt-get autoremove -y && sudo apt-get autoclean

Verify with cat /etc/os-release.

Update Firmware

You’ve come this far, might as well get the latest firmware.

$ sudo rpi-update