1. Find the manifest of the version you wish to download. I made a list of manifests from 3.3.3-12.0.1 here: https://docs.google.com/document/d/1M-6o7qo_aYz3Wj6sg-bKGq1dFx-BbR0AT4bkaoSCS7Q/edit?usp=sharing If you need to find other versions, the link to the patch notes are in the google doc.

2. Press windows key and r.

3. Enter steam://open/console into the prompt that comes up.

4. Enter the command: download_depot 700330 700331 <manifestID>. Enter the manifest of the version you wish to download into <manifestID>. For example, if you wanted to downpatch to version 11.1.5, you would enter the command: download_depot 700330 700331 3274841123143224184

5. Wait for it to download. This may take a while. Once it is downloaded, it will give you a prompt saying, "Depot download complete."

6. To find the files, navigate to your Steam folder and go to Steam > steamapps > content > app_700330 > depot_700331 If you wish to download multiple versions, make sure to move the SL files out of the folder.

Note: If you get a pop-up that says "can not connect to SL-AC" or similar and force closes the game, go to Steam>steamapps>common>SCP Secret Laboratory and copy SL-AC.dll from the folder and replace it in your downpatch folder.

Some versions (12.0.1) of the game will refuse to boot with the latest SL-AC file. In these cases, disconnect from the internet and the game should function properly. Please let me know if you find other workarounds.