Steam will automatically download the older files. You can now launch the game as version 1.5.6. On GOG (Offline Installers)
Steam allows you to "roll back" to version 1.5.6 easily using the built-in legacy branch: Open your and right-click on Stardew Valley . Select Properties from the dropdown menu. Click on the Betas tab on the left. In the "Beta Participation" dropdown, select legacy_1.5.6 . stardew valley 1.5.6 download pc
Updated Steamworks.NET to version 20.0.0 for better Steam integration. Why Stick with 1.5.6? Steam will automatically download the older files