Update information is outdated

You'll see this kind of message when one or more of your sources fails to update. Sometimes this is because your system can't get to one of the sites to download repository information (this is usually because of an outage at the site in question, a problem with an internet link somewhere along the path to the site, or a problem with DNS). Here it looks like you can't confirm the identity of the site, so Ubuntu doesn't think it is safe to use this source.

Based on your update output, you'll need to get the correct identity key for tuxfamily.org, or comment it out of your sources.

See these answers for explicit help: How do I fix the GPG error "NO_PUBKEY"?

run

  sudo aptitude

press u

then g

accept the changes if any (which means some dependencies have to be resolved)

and g again if necessary