404/Untrusted packages on apt-get

Hi, I'm very new to Ubuntu, so I respectfully ask for your patience. When I try to do apt-get install, apt-get upgrade or apt-get update, I'm getting errors like the following:

Err http://security.ubuntu.com saucy-security/main Sources
  404  Not Found [IP: 2001:67c:1360:8c01::19 80]

And the associated waing:

W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/saucy-security/main/source/Sources  404  Not Found [IP: 2001:67c:1360:8c01::19 80]

http://blogs.unity3d.com/2015/08/26/...now-available/

And I'm getting similar errors. Also, there's a dependency on lib32stdc++6, and it seems to have the same kinds of errors there as well.

Thanks for your patience.