Hallo,
is a direct upgrade from buster to bullseye possible?
Regards
Niwre
yes
Joulinar
Thank you for your prompt replay.
Are there any instructions I can use to do this?
Regards
Niwre
Theoretically yes. Depending on your installed software, additional steps might be needed. Could you share what you have installed.
As well we are preparing a blog post on how to move from Buster to Bullseye.
Joulinar
I only have WireGuard and PiHole and I installed the kernel 5.10.
First think. Whatever you do, you will do it on your own risk and we recommend to perform a full backup of your system. If possible clone your entire SD card 
ok WireGuard is not an issue. For PiHole some additional steps are needed to get the web interface back. AdBlocking and DNS resolution should be working without issues. Just the web site require some tweaks as PHP + web server will be upgrade to next version.
basically these are the main steps
sed -i 's/buster/bullseye/' /etc/apt/sources.list
sed -i 's/buster/bullseye/' /etc/apt/sources.list.d/*.list
sed -i 's|bullseye/updates|bullseye-security|' /etc/apt/sources.list
apt update
apt upgrade
apt full-upgrade
apt autopurge
/boot/dietpi/func/dietpi-obtain_hw_model
. /boot/dietpi/func/dietpi-globals
What is the web server you are running? Lighttpd? If yes I could list additional steps.
Joulinar
yes, my web server is Lighttpd.
Regards
Niwre
Joulinar
I have carried out the steps as specified, but nothing is installed with apt full-upgrade. Shouldn’t that be the upgrade to Bullseye?
in this case try afterwards
apt remove --purge php*
dietpi-software reinstall 93
sed -i 's/php7\.3/php7\.4/' /etc/lighttpd/lighttpd.conf /etc/lighttpd/conf-available/15-fastcgi-php.conf
sed -i '/^compress\./d' /etc/lighttpd/lighttpd.conf
sed -i '/"mod_compress"/d' /etc/lighttpd/lighttpd.conf
systemctl restart lighttpd
hopefully it’s all, we did not had that much upgrades so far. Could be still something missing.
usually apt upgrade should have done already the package update. At least it should have displayed around 200+ packages I guess
Joulinar
The following packages are said to be missing when I run apt update, but apt upgrade doesn’t update any of them. 
firefox-esr/stable-security 78.13.0esr-1~deb11u1 amd64 [aktualisierbar von: 78.13.0esr-1~deb10u1]
perl-base/stable-security 5.32.1-4+deb11u1 amd64 [aktualisierbar von: 5.28.1-6+deb10u1]
perl/stable-security 5.32.1-4+deb11u1 amd64 [aktualisierbar von: 5.28.1-6+deb10u1]
I though you have installed PiHole + WireGuard only. Where does FireFox is coming from?
Joulinar
I don’t remember installing Firefox, nor do I need Firefox.
can you have a look if it is installed via dietpi-software?
Joulinar
apparently i installed firefox once because it shows up. However, the first installation was a long time ago and I actually only use WireGuard and piHole.
Sorry 
I looked directly under the available software, there is no x for Firefox.
ok it is not installed via dietpi-software. Let’s remove it if not required.
G_AGP firefox-esr
Let’s check the system afterwards
apt update && apt upgrade -y
Joulinar
then unfortunately there are still two packages left that will not be updated.
perl-base/stable-security 5.32.1-4+deb11u1 amd64 [aktualisierbar von: 5.28.1-6+deb10u1]
perl/stable-security 5.32.1-4+deb11u1 amd64 [aktualisierbar von: 5.28.1-6+deb10u1]
can you share full output of
apt update
apt upgrade
apt full-upgrade
apt autopurge
Joulinar
I had just logged out of the console and then logged back in again, now it seems to work, I see loads of updates. I’ll let that go through and then I’ll get back to you. Thank you for your support.