@davidjryan There are also the instructions here: https://github.com/Duet3D/DuetSoftwareFramework/wiki/SBC-Setup-Guide#unstable-package-feed
Though I think they are the same is in the wiki.
Are you on Debian Buster? You should be on Bookworm to run 3.5 or newer. We recommend starting with a new DuetPi image, as trying to upgrade from Buster to Bookworm breaks too much. See https://docs.duet3d.com/en/How_to_guides/SBC_Buster_to_Bookworm
If you're still having problems, and reverting to an older version, you probably need to run the 'Downgrade packages' instructions: https://docs.duet3d.com/en/User_manual/Machine_configuration/DSF_RPi#downgrade-packages or https://github.com/Duet3D/DuetSoftwareFramework/wiki/SBC-Setup-Guide#downgrading-to-stable-packages
Ian