You can do it - but - you need to have kept the earlier 3CX software, as the link to the software download on the 3CX website is always the same link, but will always give you the most recent version to download i.e. unless you archive the software yourself, it's not easy to get earlier versions.
You may also have problems regarding the backup file ... It seems OK to upgrade 3CX then load a back-up, but I don't know how it would work if you downgraded 3CX then restored what would be a back-up created under a later 3CX version. To be sure - you would need to restore your configuration from an earlier back-up which was made under the 3CX version that you are downgrading to ...
I've had to do this once when I found that the back-up I had taken prior to an upgrade, was corrupt. It worked - but fortunately, I wasn't having to go back in history too far - and therefore, was not loosing any recent configuration changes ...