see this thread for a listing of extensions that bundle other extensions:
In the case of installing an extension that bundles others, the stand alone versions should not be installed.
so for example, with Design Extender, it replaces the stand alone versions of the following extensions:
-- CSS Sculptor
-- CSS Menu Writer
-- Framework Builder
-- PowerCMS Builder
-- Pro Maps for Google
-- Site Import
-- Site Sculptor
-- Social Media Toolkit
the stand alone versions of those extensions should not be installed, you only need to install Design Extender.
CSS Form Builder is not a product that is bundled into design extender, so you will need to uninstall the previous version, then install the updated version that was recently released.