Raspbian OS is the official OS for the Raspberry PI, following is a guideline that needs to be executed to update the OS from stretch to Buster which is the latest distro.

• Ensure to backup the SD card with image copy (win32Diskimage or dd)

Update all packages on the current Raspbian (stretch)

sudo apt update
sudo apt dist-upgrade -y

Preparing updates to Stretch

• Modify the file /etc/apt/sources.list and /etc/apt/sources.list.d/raspi.list
• change the word “stretch” to “buster” distribution

sudo nano /etc/apt/sources.list
sudo nano /etc/apt/sources.list.d/raspi.list 

Download and Update the OS

Around 1.5GB of packages will be downloaded and might take some time for installation (est. 6-8hours)

Additional prompt will be required during installation to override default configuration

sudo apt update -y
sudo apt dist-upgrade 

Cleaning Up

Once the Update is completed, remove some older packages that are not necessary

sudo apt autoremove -y

Reboot

The last step is to restart the Raspberry pi and checks if all services are correctly started after reboot

sudo restart