aftensleuk
I just made the Buster-based image the default download for RPi yesterday. But dietpi-update will never do distro upgrades. It is simply too risky, many things can fail, so this is out of our support and btw out of support on every Debian derivative 
.
But after doing at best a full SDcard clone, you can do it yourself:
sed -i 's/stretch/buster/' /etc/apt/sources.list
sed -i 's/stretch/buster/' /etc/apt/sources.list.d/*.list
rm -f /etc/apt/sources.list.d/dietpi-php.list
rm -f /etc/apt/trusted.gpg.d/dietpi-php.gpg
rm -f /etc/apt/preferences.d/dietpi-{php,openssl,xrdp}
/boot/dietpi/func/dietpi-set_software apt-cache clean
apt update
apt upgrade
apt full-upgrade
apt autopurge
/boot/dietpi/func/dietpi-obtain_hw_model
. /boot/dietpi/func/dietpi-globals