Big mess. The easiest is to get rid of everything and not rely entirely on the Debian packages. I had to gather some info around and I ended up with the following.
Beware, all existing kicad executable and configuration will be wiped out, as I think that 5.1 does not play well with former versions. Especially, libraries and footprints do not show up after installation. This also makes it annoying to import a former KiCad project in 5.1. I had none so I "simply" started from blank projects, as KiCad is much easier to design new packages and footprint than Eagle CAD I was using so far.
Beware, all existing kicad executable and configuration will be wiped out, as I think that 5.1 does not play well with former versions. Especially, libraries and footprints do not show up after installation. This also makes it annoying to import a former KiCad project in 5.1. I had none so I "simply" started from blank projects, as KiCad is much easier to design new packages and footprint than Eagle CAD I was using so far.