I got around this problem by adding the line "deb http://packages.linuxmint.com/ qiana main upstream import" to /etc/apt/sources.list and then attempting the upgrade. This worked for me, but I would strongly recommend backing up your system before trying it in case of disaster.
I got around this problem by adding the line "deb http:// packages. linuxmint. com/ qiana main upstream import" to /etc/apt/ sources. list and then attempting the upgrade. This worked for me, but I would strongly recommend backing up your system before trying it in case of disaster.