How I got apt update and install to work (with Debian Jessie retired)

I followed the steps in the first message but see all these kind of errors. Why does apt-get attempt to install the deb8u1 version of these packages if deb8u2 is already installed?  This type of error is seen for most package installs. 
Looks like something is defaulting to deb8u2 while something else is defaulting to deb8u1 packages...

root@steammills:/data# apt-get install gcc-4.9
Reading package lists... Done
Building dependency tree
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 gcc-4.9 : Depends: cpp-4.9 (= 4.9.2-10+deb8u1) but it is not going to be installed
           Depends: gcc-4.9-base (= 4.9.2-10+deb8u1) but 4.9.2-10+deb8u2 is to be installed
           Depends: libgcc-4.9-dev (= 4.9.2-10+deb8u1) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.