How To: Downgrade Windows 10 version 2004 to version 1909
To downgrade 2004 to 1909, follow these steps

Start menu > Settings > Update and Security > Recover Tab (if you can see an option there) > Go back to previous version.
You should be able to see it within 10 days after the upgrade.

If you are beyond the 10 day limit of being able to use the recovery tool, the only other method to go back to 1909 is to use the Microsoft Media Creation Tool for 1909. Be advised that this will basically install Windows 10, 1909 on top of your existing Windows. Your files and data should be safe according to Microsoft, however, you will most likely have to reinstall your applications.

Link to the Media Creation Tool 1909 is here -