You could update Spine to at least 4.0, open your project in it, then export a json for 3.6, but this would smoke any curves you had in your project.
A second more complex solution is explained in this thread:
https://esotericsoftware.com/forum/d/12582-open-spine-animation-in-to-previous-version/

rhey007 Could you please explain what you mean by "not working"? Does the import result in an error? If you mean that some of the animations are wrong or the same skeleton cannot be reproduced, this may be a case that cannot be resolved as those are not guaranteed when downgrading.

If you saw any error messages, please let us know what they were.