There is/was a bug whereby the releases file does not get updated, the current workaround is to remove the contents of /var/lib/apt/lists (or for the more cautious, move it out the way) and rerun apt update which will rebuild the releases file…
sudo mv /var/lib/apt/lists{,.old}
sudo apt update
Now try upgrading through whatever means you usually use. If all went well you can remove the old lists directory…
sudo rm -rf /var/lib/apt/lists.old