It’s at this point I must say I’m indebted to StepS’s guide on downgrading Beat Saber, without which I would have never known that this sort of thing was possible. That guide has additional reading into this sort of technique, so please check it out if you’re interested in learning more.
1) Determining The Codes Needed to Download
The App ID for Umurangi Generation is 1223500. The Depot ID is 1223501. However, the Manifest ID is different for each version, and this is what we’ll use to download the specific version we want. I mentioned earlier that the bug appears to be introduced in the 2022 updates, possibly the June update but I’m not 100% sure. To be safe, I chose the 2021-Oct-12 update, which has a Manifest ID of 5540599311058635572
2) Using the Steam Console
Steam should switch to a console section with an entry field at the bottom. The format we use is “download_depot <App ID> <Depot ID> <Manifest ID>”. So for the 2021-Oct-21 version the code is “download_depot 1223500 1223501 5540599311058635572”
Press Enter to begin your download. Steam will not display download progress, but it should be running the the background. Wait until you see the message that begins with “Depot download complete”.
Take note of the file path, you’ll be using it shortly.
3) Managing Files
Now go to where you downloaded the old version. You can copy the address you received from your Steam Console and paste into the address bar of your file explorer. Once you are there, move everything into your steamapps\common\Umurangi Generation folder.
Congratulations, you have downgraded your game. In your Steam program go back to your library and play your game like normal. Re-earn the Speed runner achievement, and once you are finished with this version of the game, either replace your game files with your back-up, or have Steam re-download the current version to restore everything to normal.