Ubuntu apt-get update error 404 Not Found

All you need to do to resolve this problem is to modify your /etc/apt/sources.list file.  replace any lines like below,

deb http://us.archive.ubuntu.com/ubuntu/ precise universe
deb-src http://us.archive.ubuntu.com/ubuntu/ precise universe
deb http://us.archive.ubuntu.com/ubuntu/ precise-updates universe
deb-src http://us.archive.ubuntu.com/ubuntu/ precise-updates universe

deb http://security.ubuntu.com/ubuntu precise-security main restricted
deb-src http://security.ubuntu.com/ubuntu precise-security main restricted
deb http://security.ubuntu.com/ubuntu precise-security universe


to


deb http://old-releases.ubuntu.com/ubuntu/ precise universe
deb-src http://old-releases.ubuntu.com/ubuntu/ precise universe
deb http://old-releases.ubuntu.com/ubuntu/ precise-updates universe
deb-src http://us.archive.ubuntu.com/ubuntu/ precise-updates universe
deb http://old-releases.ubuntu.com/ubuntu precise-security main restricted
deb-src http://old-releases.ubuntu.com/ubuntu precise-security main restricted
deb http://old-releases.ubuntu.com/ubuntu precise-security universe



All that's happened is that the installed ubuntu version has fallen out of support, so you need to point your apt sources at the old-releases repo. The file shown just has the original sources commented out and a new old-releases.ubuntu.com location added in.

All the other forum posts about moving sources.llist around, running sed commands against your sources and all that junk doesn't work, or at least didn't for me.. I don't even see that the sed replacement would have replaced anything in my case..