u/pajicadvance23

1. Open your browser, paste steam://nav/console into the address bar and hit Enter
2. You will be prompted to open Steam, click Open Steam and Steam will open with a command window
3. Enter download_depot 489830 489831 8442952117333549665 and wait for the download to finish
4. Enter download_depot 489830 489832 8042843504692938467 and wait for the download to finish
5. Enter download_depot 489830 489833 1914580699073641964 and wait for the download to finish
6. For each depot you downloaded there will be a location printed out, usually it's C:\Program Files (x86)\Steam\steamapps\content\app_489830\depot_[number]
7. Create a new folder anywhere where you want your 1.6.1170 installation to be
8. Copy the files from all three depot folders into your new folder ("combine" them essentially)
9. Launch Skyrim using exe files from that folder instead of launching through Steam
10. To make Steam launch 1.6.1170, open Steam, right click Skyrim Special Edition in your game list, click Properties, and add "[your 1.6.1170 folder]\SkyrimSE.exe" %command% to Launch Options

edit: As someone in the thread mentioned, you definitely can just take the new files and put them where your game is already installed without creating a new folder, however, make sure to apply one of the methods to prevent Steam from updating the game if you do that.