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.
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. 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.