You can change the Spine version in Spine's settings:
Settings - Spine User Guide
The list shows the versions that you've used previously. If you want a version not listed, click Other...
and type it in. The full list of versions is here:
Spine: Changelog
I'm afraid the trial version cannot save and never has been able to. Also, the trial is always the latest version (latest beta when a beta is active), an older version cannot be chosen.