If you are looking for Update 84 specifically to maintain compatibility with older mods or for speedrunning purposes, you can manually revert your game version using the Steam Console.
: Your console will prompt you to download the update before launching the game while connected to the internet. Manual Downgrade to Update 84 (Advanced Users)
: Once the download is complete, Steam will provide a file path (usually under Steam\steamapps\content\app_264710 ). You can move these files to a separate folder to run that specific version.