You can’t really downgrade an existing version of Mendix. You could try to get the previous version and work from there to implement the changes once more.
1. Mendix Studio Pro - > VersionControl -> Manage Branch Lines : Create a Branch from the version before your last commit occurred on Mendix 9.9.1 on Version Control.
2. Mendix Studio Pro - > VersionControl -> Download from Version Control Server : Select the newly created Branch above and download it on your local computer. Now this your App on Mendix 9.8
3. You can continue to use the Branch for further development or Merge it with the Main Branch and use it.