Ubuntu 16.04 Software Update GUI is hanging and I'm getting this message in the terminal

[Ralf Mardorf]

I copy and pasted

  W: http://dl.google.com/linux/chrome/deb/dists/stable/Release.gpg:
  Signature by key 4CCA1EAF950CEE4AB83976DCA040830F7FAC5991 uses weak digest algorithm (SHA1)

into a search engine's search bar and the first hit was

http://askubuntu.com/questions/760796/how-to-fix-apt-signature-by-key-uses-weak-digest-algorithm-sha1-after-install

It explains everything you need to know.

Regards,
Ralf