Starting with release 6.2.1711 Ubuntu packages are now hosted on Launchpad.
Stable: https://launchpad.net/~ngsolve/+archive/ubuntu/ngsolve
Nightly: https://launchpad.net/~ngsolve/+archive/ubuntu/nightly
Please remove the old repository before updating, instructions to do so are for instance here:
To activate the new repository follow the instructions on the Downloads page