chrishamm replied via DSF issue board confirming that M26 is buggy and will be fixed in next release.

Well, thanks everybody for your input - finally got my stuck RRF version upgraded properly with the help of BOSSA. Went for the 3.0 stable btw.

@Danal I am about to give up trying to find a workaround with M997. Will try with bossa/bossac soon and report here how it went. Ty for your suggestion and guide.

@blt3dp now M997 finishes without errors, but still end up with same "3.0beta12+1".

Everything else seems to be updating without problems via apt-get update/upgrade.

Here's the list of files in my /sys/, maybe one of you can spot I am missing something or there's anything which doesn't belong there:

.      config.g     Duet3Bootloader_EXP.bin   Duet3Firmware_MB6HC.bin  Duet3iap_spi_MB6HC.bin  Duet3_SBCiap_MBP05.bin  filaments.csv  homey.g    pause.g   stop.g    tpre0.g
..     config.json  Duet3Bootloader_TOOL.bin  Duet3Firmware_MBP05.bin  Duet3iap_spi_MBP05.bin  Duet3_SDiap_MB6HC.bin   homeall.g      homez.g    resume.g  tfree0.g
bed.g  configOld.g  Duet3Firmware_EXP3HC.bin  Duet3iap_sd_MB6HC.bin    Duet3_SBCiap_MB6HC.bin  dwc2settings.json       homex.g        iap4e.bin  sleep.g   tpost0.g

@dc42 got both IAP files in sys.

Now, while calling M997 to update, I actually get an error:

Error: Failed to create directory 0:/sys
Error: Failed to create folder 0:/sys while trying to open file 0:/sys/filaments.csv

Ideas anyone?

@jay_s_uk I'll start reading on how to do that, ty
@deckingman tried that, still same.

Ok, got some extra info I failed to mention which may be of relevance: I have two expansion boards attached to my duet. Those I managed to update to 3.0RC2 2019-12-28b4 - (that's what I get if I sent M115 B1 / M115 B2).

Well in that case I am confused about what am I doing wrong. Very open to suggestions

apt-get update
apt-get upgrade duetsoftwareframework

seemed to be working for most part, until I got error:

Setting up duetcontrolserver (1.2.2.0) ...
Setting up duettools (1.2.2.0) ...
Setting up reprapfirmware (1.2.2.0-1) ...
[error] Failed to connect to Duet
   System.IO.IOException: Error 16. Cannot put line into event mode.
   at LinuxDevices.InputGpioPin..ctor(String devNode, Int32 pin, String consumerLabel) in /home/christian/duet/DuetSoftwareFramework/src/LinuxDevices/InputGpioPin.cs:line 60
   at DuetControlServer.SPI.DataTransfer.Init() in /home/christian/duet/DuetSoftwareFramework/src/DuetControlServer/SPI/DataTransfer.cs:line 65
   at DuetControlServer.SPI.Interface.Init() in /home/christian/duet/DuetSoftwareFramework/src/DuetControlServer/SPI/Interface.cs:line 65
   at DuetControlServer.Program.Main(String[] args) in /home/christian/duet/DuetSoftwareFramework/src/DuetControlServer/Program.cs:line 92

then did

apt reinstall reprapfirmware

and while

Setting up reprapfirmware (1.2.2.0-1) ...

no errors occured. So now I have:
DSF: 1.2.2.0
RRF: 1.2.2.0

Sent M997, said updating + restarts. M122 says I am still on "RepRapFirmware for Duet 3 MB6HC v0.6 or 1.0 3.0beta12+1 (2019-11-11b1) ".

Uploading "Duet3Firmware_MB6HC.bin" for either RC1 or RC2 as system file initializes update, restarts + M997 ....aaaand still says I am on 3.0beta12+1.

Whats up with that?

@Danal, if I am getting the "unstable" versions of DSF+RRF via apt-get upgrades means I am on "beta" feed, right?

did that, I get:

reprapfirmware is already the newest version (1.2.1.0-1).

Thanks to @Dougal1957 and @blt3dp pointing me at right direction - updated DSF to 1.2.1.0.

So now uploading "Duet3Firmware_MB6HC.bin" as system file starts update without errors. However, after installation it still shows that Firmware is : RepRapFirmware for Duet 3 MB6HC v0.6 or 1.0 3.0beta12+1 (2019-11-11b1)

What am I missing here?

I am pretty sure I am up to date with my Pi 4. The file(s) I am missing either came on sd with my Duet3 or were supposed to be generated/installed with an update to my current 3.0beta12+1 version.