1) Download a FULL package of 5th patch for your game version from the official site or other resource. The file named stk-*your_game_version_description*-patch-any-5.exe - it's size around 60 MB.
2) Start Regedit by pressing Win+R, typing "regedit" word without quotes, navigate in registry to the path: HKEY_LOCAL_MACHINE \ SOFTWARE \ GSC Game World \ STALKER-SHOC (for Windows Vista/7/8 x64 version, this path will be HKEY_LOCAL_MACHINE \ SOFTWARE \ Wow6432Node\ GSC Game World \ STALKER-SHOC ), there you find the option named "InstallVers". Its value is must be 1.0006 - change it to the 1.0004
3) Now run the downloaded 5th patch - it will normally patch game to version 5.
4) Now you have proper game version and can start mod installation.
How to roll back game version from 1.0006 to 1.0005
This guide will help you to roll back game version from 1.0006 to 1.0005 if you want to install OGSE 0.6.9.2 R2 but have a latest game release.
Posted by KamikaZzeRu on - Basic Installers
I tried this with my Steam version, keeps updating over steam every time I close the game, steam has 3 options for updates, update always, priority update, or update when opening game, either way I am screwed I guess.
You can get the game on GoG and it will roll back just fine, I was in the same boat till i found that version.