If you are using Steam, right-click Stardew Valley > Properties > Updates > Set to "Only update this game when I launch it." Then, always launch via the .exe in the folder, not the Steam Play button.
Steam usually forces updates, but Valve built a "Beta" system for legacy versions. Note: This only works if the developer enables it. ConcernedApe has enabled previous versions.
How to find the Steam compatibility download link: stardew valley compatibility version download link
Caveat: You must own the game, and you cannot use Steam Cloud saves reliably between versions.
Once you have your Stardew Valley compatibility version download link, follow these rules to avoid losing your farm. If you are using Steam, right-click Stardew Valley
Q: Is it legal to use a compatibility version? A: Yes, as long as you own a legal license (Steam, GOG, Xbox, Switch). Downloading the game file from a torrent is piracy; rolling back via official tools is not.
Q: Will I lose my multiplayer ability? A: Yes. A player on v1.5.6 cannot host a game for someone on v1.6. Everyone must be on the exact same compatibility version. Steam will immediately download the older version
Q: Can I download Stardew Valley 1.0 (the original release)? A: No. ConcernedApe has not released public depots for versions older than 1.2. You cannot legally download the launch-day version anywhere.
