Hi.
When you run OpenFL, it creates a "buid" file (no extension, I believe) which has the APK version number. Your Android device is complaining that you're trying to shift from a higher-version to a lower version.
I think the correct/intended workflow is to keep and check in the build file (I don't do this either). Instead, you can always uninstall and reinstall your app.
This may also bite you once your app is uploaded to the Google App Store. You can always change the value of the build file by hand (eg. upload your first version to the store as #1, or put a higher number and rebuild.)