If you're on a windows machine, 1) turn off the steam autoupdate function, 2) use the "restore previous versions" function to revert back to an older version of the CK2 folder inside steam, 3) copy this folder somewhere else on your computer.
Now if you want, you can let steam autoupdate your main copy of CK2, while keeping the older version in a different folder.
Now if you want, you can let steam autoupdate your main copy of CK2, while keeping the older version in a different folder.