I presume your question relates to your post
b4j cannot open visual designer. says choose program to open fxml files. where you are looking to be able to open the fxml layout files from existing projects.
Importantly you don't need to "downgrade" to an earlier version of B4J and really that is not the "problem".
I have several legacy projects using fxml layout files which I don't want to convert using the internal designer and indeed the option the option to open the fxml files directly from within B4J has been removed in recent versions, but you can still open them by going to where they are located in the Files folder of the project.
Presumably you still have JavaFX Scene Builder - the app required to edit the fxml files - still installed and so double clicking on the fxml file should open it in JavaFX Scene Builder. If the file type fxml is not associated with JavaFX Scene Builder then you should do that via Windows setup so it does open automatically.
If for some reason you have uninstalled JavaFX Scene Builder then you can find an installer here:
https://www.oracle.com/java/technologies/javafxscenebuilder-1x-archive-downloads.html I still use version 1.1.