Thanks kzerse.
I'll explain how it works start to finish, for all OSes. This file stores which update Spine will use when it starts:
Windows: <user home folder>\Spine\version.txt
Mac: <user home folder>/Library/Application Support/Spine/version.txt
Linux: <user home folder>/.spine/version.txt
You can create it the file doesn't exist. You can edit the file and change the update version, when you restart Spine it will use the update you specify. Be sure to specify the name of one of the updates you have, which are the folders you find here:
Windows: <user home folder>\Spine\updates\
Mac: <user home folder>/Library/Application Support/Spine/updates/
Linux: <user home folder>/.spine/updates/
If you specify an invalid update version in version.txt or you delete version.txt, then Spine will download the latest update.
Setting the update version in version.txt is the same as choosing a version from the Settings dialog in Spine. This means Spine will run that update and not download the latest.