The sequel of the legendary wargame with a complete graphics and interface overhaul, major new gameplay and design features such as full naval combat modelling, improved supply handling, numerous increases to scenario parameters to better support large scenarios, and integrated PBEM++.
download the installer 15th of Nov 2017, which is the first you need, as it is the latest version of the game and will be better than the one most of the buyers will have eg 1.0 etc
then once the installer is finished, either use the auto updater to add in the latest patch, or download the patch from above 15th of Oct 2018
primary or secondary only means if one is slow with too many on it, you can always download from the other one, so it's the same file, just on 2 different addresses
install it to where ever you like, as long as it's not the root directory etc and on any drive, as long as it's a local one, and not a hotswap eg unplug able one / or none local
always say yes, as it only over writes if you already have it, but to be on the safe side and it doesn't take more than a 30 seconds, i always do it, no matter what