officially a downgrade is not supported, and therefore not tested. Thus we do not have official documentation about this process.
May I ask why you need to downgrade your system? Do you encounter problems with a new version?
For compliance purposes. Is there any written instructions?
as downgrading isn't supported, this will require support's approval. I suggest opening a ticket with support. Downgrading in general is a reimage of the appliance and to use a configuration backup of the config.