Jimmy, the guide is correct. The line in your sources.list should look something like that:
deb http://archive.ubuntu.com/ubuntu/ focal-proposed main universe
Please note that it is safer to selectively update the packages you are interested in (in this case, thunderbird* and possibly enigmail) and then comment out again that line, to avoid pulling in all of focal-proposed, with unrelated updates.
Jimmy, the guide is correct. The line in your sources.list should look something like that:
deb http:// archive. ubuntu. com/ubuntu/ focal-proposed main universe
Please note that it is safer to selectively update the packages you are interested in (in this case, thunderbird* and possibly enigmail) and then comment out again that line, to avoid pulling in all of focal-proposed, with unrelated updates.