In-Place Upgrade
First check if all your applications are supported under Raspbian ‘Buster’.
Open a terminal, and type the following:
Update Raspberry firmware :
sudo rpi-update
Change sources files from Stretch to Buster :
grep -rl stretch /etc/apt/ | sudo xargs sed -i ‘s/stretch/buster/g’
This finds all the source definitions for the apt package manager referring to ‘Stretch,’ the previous version of Raspbian, and replaces them with ‘Buster’.
Once finished, update package lists and start upgrade, type:
sudo apt update && sudo apt dist-upgrade
Note that this process can take several hours to complete. During the process check frequently if user interaction is needed. The upgrade process will ask you several times what to do whith existing configuration files.
When it has finished, reboot the Raspberry Pi.
Finally, tidy up the now-unsupported applications which have come through from Raspbian ‘Stretch’ with the following terminal command:
sudo apt purge timidity lxmusic gnome-disk-utility deluge-gtk evince wicd wicd-gtk clipit usermode gucharmap gnome-system-tools pavucontrol
or just use : sudo apt autoremove -y