Here's how you can download older versions of files in a game from Steam using SteamDB to navigate the game's patch history. The example here is reverting Skyrim SE's 1.6.640 patch so we're back to the 1.5.97 version of SkyrimSE.exe.
You can find all of the available depots for a given game by looking up its appid as follows: https://steamdb.info/app/489830/depots/ - 489830 here is the appid for Skyrim SE.
First, you have to find what changed, so click the History button down the left side: https://steamdb.info/app/489830/history/
That shows the last change - Changelist #16107407 at the time of writing - as happening for depot https://steamdb.info/depot/489833/ which is Skyrim Special Edition exe and contains just SkyrimSE.exe.
If you click the History button on the left, you can see all the changes released for that depot: https://steamdb.info/depot/489833/history/