How to downgrade to an earlier game version, if you've already updated on accident

If you get caught out by the automatic update on 11 November, or if you simply decide you want to downgrade to Skyrim 1.5.97 (this the latest game version at the current point in time), but you did not make a backup, then you can download the older EXE from a Steam depot using Depot downloader
or a patcher,

To do so:

Use the downgrader or use Steam Depot downloader, or the new downloader.

==Method one==

See section "Method one"

==Method two==

See section "Method two"

==Method three==

See section "Method three"

And that's it, this way you will stay on the current version of the game and ensure compatibility with SKSE dependent mods - at least for the time being and according to what we know at this point.

One way to roll back the update, and my preferred one, is to use this simple patcher,

First you download it from here[www.nexusmods.com]

Second, you run the exe file

Thirdly you click patch and sit back while it fixes it for you.

Kudos for Lizdrfishr for finding this gem early on in this sitiuation.

You could also use the new application by Halgari (maker of the downgrade patcher)
https://www.nexusmods.com/skyrimspecialedition/mods/61756
it allows you to easily download old versions of Skyrim, caution you do need to login to Steam
so it knows you aren't pirating Skyrim.

Manifest not available fix[www.nexusmods.com] This guide explain how to revert to an older version of Skyrim after a stealthy update in spite of the locking system put in place by Steam last update. It will permit you to play until a new SKSE version is released. It is useful if you use SKSE mod incompatible with last version or if you don't want to update all your mods.

Kudos to Vlad for finding this method.

The methods listed in the guide is in case you forgot to backup and
set to update on launch while launching from SKSE64.

To backup your game, copy your Skyrim Special Edition folder here
Steam\steamapps\common\SkyrimSpecialEdition
To somewhere like documents, because it is a large folder I suggest you
archive it with an archiving tool like Peazip[peazip.github.io] or 7-zip[www.7-zip.org]

Now that things are the way they are, I strongly believe that most mods will update, so in the future after there has been updates to all the mods you want I would advise to not use these methods and instead update and find a up to date modding guide or just play the game.

Here is Arthmoor's take on things as they are now,
https://www.afkmods.com/index.php?/topic/5954-skyrim-anniversary-edition-and-you/

Lots and lots of kudos for BigBizKit, Vlad, Lizdrfishr, and all those over at Nexus Mods for suggesting various ways to save your modding.