1. Enable SSH.
2. Find the build number you want on one or more of the following pages:
https://support.steampowered.com/kb_article.php?ref=8293-PDVM-4434
http://media.steampowered.com/steamlink/06_2015/public_builds.txt
http://media.steampowered.com/steamlink/06_2015/beta_builds.txt
3. Write the desired build number to "/mnt/config/system/update_branch.txt'". e.g. for build 640 you'd run:
ssh [email protected] 'echo 640 > /mnt/config/system/update_branch.txt'
4. Go to the update menu on your Steam Link and try to update it; it should now offer to update to the firmware revision previously specified.
Technical Explanation
By default, the Steam Link will load either the public_builds.txt or beta_builds.txt list depending on the branch. If a newer build is found, then the device will attempt to find a delta image (to save time/bandwidth); if it can't find a delta image, then it will download a full factory image.
When a build number is set instead of the branch ("beta" or "public"), the device will manually search for a update file regardless of the currently installed build. It will always fail at finding a delta for a downgrade, and then fallback to a full release.
