I don't think you can, through beta's or Steam console
To go back to 1.05, have a Steam 1.08 copy of Undertale on Windows, and download the 1.08-1.05 patch file here (or directly here). Then, either download Floating IPS, or use an online patcher. The latter is recommended, but both should work fine. Then, using Flips or the site, open the data.win file in your UNDERTALE directory, and select the .bps file that you just downloaded as the patch. Then, save the patched file and overwrite the previous data.win file. Shayy made this video a while back, which should help you if you have no idea what to do.
And as always, you can join the Undertale Speedrunning Community Discord server for extra support
There is a way using the steam console, but iirc it requires changing a value in memory/hex editing.
The patch is 100% recommended though, because it eliminates the need for HxD or Cheat Engine
