Hi,
probably these 2 links will help you
https://dietpi.com/forum/t/in-place-os-upgrade-with-dietpi-platform/2866/1
https://dietpi.com/forum/t/stretch-to-buster-upgrade/3396/8
and maybe this one as well
https://dietpi.com/forum/t/qbittorrent-version-shipping-with-dietpi/2326/9
EDIT:
I would recommend to go with a new buster image because all images have been recreated