I have a subscripton to Captivate 8 how do I upgrade my subscription to Captivate 9

You have a subscription to Captivate, not to a particular version

What is bit strange is the way you have to get the version: download the trial version of Captivate 9, when you log in with your Adobe ID (which is linked to your subscription) you will be recognized as having a subscription. You can keep two versions (8 and 9), which I strongly recommend because once you converted a file to the 9 version, you cannot open it anymore with 8. It is safe before upgrading a project to keep the file saved in the previous version as well.

Be sure to check the version number: with the hotfix released soon after the first release, you should be on 9.0.0.230. If that is not the case, have a look at this KB and install the hotfix manually:

https://helpx.adobe.com/captivate/kb/hot-fix-captivate-9.html