Check your current OS version:

uname -a

cat /etc/debian_version

Prepare

Get up to date.

sudo apt-get update
sudo apt-get upgrade
sudo apt-get dist-upgrade

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

Optionally upgrade the firmware.

sudo rpi-update  

Prepare apt-get

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. Run 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.

grep -lnr stretch /etc/apt  

Speed up subsequent steps by removing the list change package.

sudo apt-get remove apt-listchanges

Do the Upgrade

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

cleanup old outdate packages.

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

Verify with:

cat /etc/os-release.

Update Firmware

You have come this far, might as well get the latest firmware:

sudo rpi-update    

Have fun with your new updated Pi! 😉