After Skyrim AE Update
This guide will tell you how to get started with the nightly builds after the Skyrim Anniversary Edition update

Overview

This guide exists due the fact that Bethesda released updates to Skyrim SE, which leads to the error "This version is not supported".
The Elder Scrolls V: Skyrim Special Edition - The Elder Scrolls V: Skyrim Anniversary Edition - Update 1.6 - Steam News
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Checking the installed Version

First we check the version of Skyrim SE which you have currently installed.
Right click Skyrim Special Edition
Click Local Files -> Browse...
Right click SkyrimSE.exe and then Properties
Check the File version
If your File Version is 1.5.97 you don't need to do anything, since you're already on the correct version of Skyrim SE. Follow the normal guide for getting started with the Nightly Builds below.
If this is not the case, you need to verify your game files and then downgrade Skyrim to the correct version. More in the next step.
Skyrim Script Extender has changed how it loads DLLs in the latest build. You need to downgrade to skse64_2_00_19 to continue using it with Skyrim Together. Archived versions of the previous builds are available for download on their website:

Downgrading the game

Updating the game

To make downgrading Skyrim SE easier, we fully update it first and verify the game files. For this we once again go to the Game properties -> Local Files as in the first step. Now click on Verify integrity of game files...
Click Local Files -> Verify integrity of game files...
After the update was fully downloaded, check your file version like we did in the first step of this guide. As of now (2022.01.07) the file version should be "1.6.353".

Downgrading the game

The next step is to download the Patcher from nexus mod. Be sure to manually download the FullPatcher.exe. Should the version of the FullPatcher.exe be lower than your game version, your game received another updated and the Patcher hasn't been updated yet. Is your game version instead lower, you need to update your Skyrim SE to the latest version. https://www.nexusmods.com/skyrimspecialedition/mods/57618
If you have downloaded the FullPatcher.exe in the past, you most likely have to redownload it again since it was updated to the new Skyrim SE update. The older downloaded Patcher won't just work with a newer Skyrim SE.
Click manual download of FullPatcher.exe
After downloading the FullPatcher.exe, you need to run it. You will most likely get this message saying that you need to select Skyrim SE manually.
Click the folder symbol and navigate to your SkyrimSE.exe and select it. The SkyrimSE.exe can be found just like we did in step one. Now just press Open an then Start Patching.
Click the folder Icon and select SkyrimSE.exe
As soon as the Patcher shows the following, you can follow the Getting started with Nightly Builds guide. Have fun.
Successfully patched gamed