Note: If an explicit 1.5.6 branch is not visible in the dropdown, you can use external community tools like to safely fetch the exact manifests of the 1.5.6 patch directly from Steam's official databases. Setting Up Mods for Stardew Valley 1.5.6
If you're not using a digital storefront or prefer a direct download, you can purchase Stardew Valley from the official website. The website often provides a link to the latest version of the game, which will include the 1.5.6 update if available.
Head to the official SMAPI website or its GitHub release page. Look through the "Older Versions" archive.