Hi MIRGERVIN
I assume that you are referring to the latest beta patch? Download the patch from
here, then extract it to somewhere convenient (like your Desktop). The extracted folder (‘Parts’) must then be copied to your game folder to over-write the existing one. If you wish to make a backup, simply copy the existing ‘Parts’ folder to some backup location before over-writing it. If you then wish to revert to the previous version, rename the updated ‘Parts’ folder to something else (like ‘Parts18301’) and copy the original back. You can then swop between the two versions by renaming the folders in turn. You can also revert to the latest official version using Steam to verify the game files.
It is also a good idea to delete the contents of the cache folder when updating to the beta version (and back again if required). The cache is in your ‘Documents\1C Softclub\IL-2 Sturmovik Cliffs of Dover\Cache\Shaders’ folder – I can’t remember the exact folder names as I am at the office at the moment. The game will re-create the cache when it is run again.