OK the hotfix url has expired. Maybe @sam_nazarko can remind us the cli commands you need. This might work: make a file
sudo nano /etc/apt/apt.conf.d/00-osmc-allow-suite-changes
with the following content
Acquire::AllowReleaseInfoChange::Suite "true";
quiet::ReleaseInfoChange::Version "true";
Then try to update-upgrade again.
But maybe you just have to reinstall Cannot update, even after hotfix - #3 by sam_nazarko