(Template of this guide copy-pasted from the Ritual of the Night speedrunning page)

To downpatch to Version 1.0...

• Turn off Steam Cloud. To do that, right click the game on Steam —> Properties —> Uncheck "Keep games saves in the Steam Cloud" (Very 1st tab). This can avoid Steam replacing your save files and ruining everything.
• Turn off auto updates when lauching the game. To do that, right click the game on Steam —> Properties —> Updates. You need "Always keep this game updated" on and NOT "Only update this game when I launch it".
• Press Windows key+R to open the run command, and type in: steam://nav/console
• Type this into the Steam console: download_depot 2369900 2369901 3697199310595088523
• After the download is complete, the Steam console will tell you its location. Copy-paste these files to somewhere convenient.
• Next, right click the game on Steam —> Manage—> Browse local files to find the 1.001 files. Simply replace these files with the 1.0 files.