Please export a json from your current version of Spine and set the version to 3.7:
Remember to also check "nonessential data".
As the warning will tell you, any data that has been changed or added in most recent versions of Spine will be lost when downgrading, so please expect for example curves to be lost when downgrading.
Next, downgrade your Spine version to 3.7 and import the json you just generated.
Save the project and you're done!